feat(CI): add a GitHub Action to lint the Markdown

Run on pull request only
Using https://www.npmjs.com/package/markdownlint-cli
Lint Markdown for rules:
- MD009/no-trailing-spaces
- MD012/no-multiple-blanks
- MD022/blanks-around-headings
- MD031/blanks-around-fences
- MD032/blanks-around-lists

4 files changed, 79 insertions, 4 deletions

diff --git a/doc/guides/compile.md b/doc/guides/compile.md
index b0dedfc8e..527868704 100644
--- a/doc/guides/compile.md
+++ b/doc/guides/compile.md
@@ -6,6 +6,7 @@ binaries.
 ## Prerequisites
 
 To compile mruby out of the source code you need the following tools:
+
 * C Compiler (e.g. `gcc` or `clang`)
 * Linker (e.g. `gcc` or `clang`)
 * Archive utility (e.g. `ar`)
@@ -18,6 +19,7 @@ the `$PATH` to compile `mruby`. We also encourage to upgrade `ruby`
 on MacOS in similar manner.
 
 Optional:
+
 * git (to update mruby source and integrate mrbgems easier)
 * C++ compiler (to use GEMs which include \*.cpp, \*.cxx, \*.cc)
 * Assembler (to use GEMs which include \*.asm)
@@ -61,6 +63,7 @@ configure the build environment for specific compiler infrastructures.
 #### GCC
 
 Toolchain configuration for the GNU C Compiler.
+
 ```ruby
 toolchain :gcc
 ```
@@ -69,6 +72,7 @@ toolchain :gcc
 
 Toolchain configuration for the LLVM C Compiler clang. Mainly equal to the
 GCC toolchain.
+
 ```ruby
 toolchain :clang
 ```
@@ -78,6 +82,7 @@ toolchain :clang
 Toolchain configuration for Visual Studio on Windows. If you use the
 [Visual Studio Command Prompt](https://msdn.microsoft.com/en-us/library/ms229859\(v=vs.110\).aspx),
 you normally do not have to specify this manually, since it gets automatically detected by our build process.
+
 ```ruby
 toolchain :visualcpp
 ```
@@ -85,6 +90,7 @@ toolchain :visualcpp
 #### Android
 
 Toolchain configuration for Android.
+
 ```ruby
 toolchain :android
 ```
@@ -95,7 +101,7 @@ in `ANDROID_STANDALONE_TOOLCHAIN`.
 ### Binaries
 
 It is possible to select which tools should be compiled during the compilation
-process. For example, 
+process. For example,
 
 * `mruby`
 * `mirb`
@@ -106,6 +112,7 @@ The configuration are done via `mrbgems`. See `Mrbgems` section.
 
 Some environments require a different file separator character. It is possible to
 set the character via `conf.file_separator`.
+
 ```ruby
 conf.file_separator = '/'
 ```
@@ -113,6 +120,7 @@ conf.file_separator = '/'
 ### C Compiler
 
 Configuration of the C compiler binary, flags and include paths.
+
 ```ruby
 conf.cc do |cc|
   cc.command = ...
@@ -128,6 +136,7 @@ end
 C Compiler has header searcher to detect installed library.
 
 If you need a include path of header file use `search_header_path`:
+
 ```ruby
 # Searches ```iconv.h```.
 # If found it will return include path of the header file.
@@ -136,6 +145,7 @@ fail 'iconv.h not found' unless conf.cc.search_header_path 'iconv.h'
 ```
 
 If you need a full file name of header file use `search_header`:
+
 ```ruby
 # Searches ```iconv.h```.
 # If found it will return full path of the header file.
@@ -149,6 +159,7 @@ When you are using GCC toolchain (including clang toolchain since its base is gc
 it will use compiler specific include paths too. (For example `/usr/local/include`, `/usr/include`)
 
 If you need a special header search paths define a singleton method `header_search_paths` to C compiler:
+
 ```ruby
 def conf.cc.header_search_paths
   ['/opt/local/include'] + include_paths
@@ -158,6 +169,7 @@ end
 ### Linker
 
 Configuration of the Linker binary, flags and library paths.
+
 ```ruby
 conf.linker do |linker|
   linker.command = ...
@@ -175,6 +187,7 @@ end
 ### Archiver
 
 Configuration of the Archiver binary and flags.
+
 ```ruby
 conf.archiver do |archiver|
   archiver.command = ...
@@ -185,6 +198,7 @@ end
 ### Parser Generator
 
 Configuration of the Parser Generator binary and flags.
+
 ```ruby
 conf.yacc do |yacc|
   yacc.command = ...
@@ -195,6 +209,7 @@ end
 ### GPerf
 
 Configuration of the GPerf binary and flags.
+
 ```ruby
 conf.gperf do |gperf|
   gperf.command = ...
@@ -203,6 +218,7 @@ end
 ```
 
 ### File Extensions
+
 ```ruby
 conf.exts do |exts|
   exts.object = ...
@@ -247,6 +263,7 @@ See doc/mrbgems/README.md for more option about mrbgems.
 Configuration Mrbtest build process.
 
 If you want mrbtest.a only, You should set `conf.build_mrbtest_lib_only`
+
 ```ruby
 conf.build_mrbtest_lib_only
 ```
@@ -259,6 +276,7 @@ See `mruby-bin-*/bintest/*.rb` if you need examples.
 If you want a temporary files use `tempfile` module of CRuby instead of `/tmp/`.
 
 You can enable it with following:
+
 ```ruby
 conf.enable_bintest
 ```
@@ -277,6 +295,7 @@ files are compiled by C++ compiler.
 
 When you mix C++ code, C++ exception would be enabled automatically.
 If you need to enable C++ exception explicitly add the following:
+
 ```ruby
 conf.enable_cxx_exception
 ```
@@ -286,20 +305,24 @@ conf.enable_cxx_exception
 If your compiler does not support C++ and you want to ensure
 you don't use mrbgem written in C++, you can explicitly disable
 C++ exception, add following:
+
 ```ruby
 conf.disable_cxx_exception
 ```
+
 and you will get an error when you try to use C++ gem.
 Note that it must be called before `enable_cxx_exception` or `gem` method.
 
 ### Debugging mode
 
 To enable debugging mode add the following:
+
 ```ruby
 conf.enable_debug
 ```
 
 When debugging mode is enabled
+
 * Macro `MRB_DEBUG` would be defined.
 	* Which means `mrb_assert()` macro is enabled.
 * Debug information of irep would be generated by `mrbc`.
@@ -330,6 +353,7 @@ directory.
 
 In cross compilation, you can run `mrbtest` on emulator if
 you have it by changing configuration of test runner.
+
 ```ruby
 conf.test_runner do |t|
   t.command = ... # set emulator. this value must be non nil or false
@@ -369,6 +393,7 @@ root directory. The structure of this directory will look like this:
 	          +- mruby
 
 The compilation workflow will look like this:
+
 * compile all files under *src* (object files will be stored
 in *build/host/src*)
 * generate parser grammar out of *src/parse.y* (generated
@@ -446,6 +471,7 @@ build directory.
 The cross compilation workflow starts in the same way as the normal
 compilation by compiling all *native* libraries and binaries.
 Afterwards the cross compilation process proceeds like this:
+
 * cross-compile all files under *src* (object files will be stored
 in *build/i386/src*)
 * generate parser grammar out of *src/parse.y* (generated
diff --git a/doc/guides/debugger.md b/doc/guides/debugger.md
index 61c0418b0..9d0c005c4 100644
--- a/doc/guides/debugger.md
+++ b/doc/guides/debugger.md
@@ -220,6 +220,7 @@ Example:
 ```
 
 Enabling all breakpoints
+
 ```
 (foo.rb:1) enable 1 3
 ```
diff --git a/doc/guides/mrbconf.md b/doc/guides/mrbconf.md
index d01eff258..2130d51c2 100644
--- a/doc/guides/mrbconf.md
+++ b/doc/guides/mrbconf.md
@@ -1,7 +1,9 @@
 # mruby configuration macros.
 
 ## How to use these macros.
+
 You can use mrbconfs with following ways:
+
 * Write them in `mrbconf.h`.
  * Using compiler flags is preferred  when building a cross binaries or multiple mruby binaries
  since it's easier to use different mrbconf per each `MRuby::Build`.
@@ -11,7 +13,9 @@ You can use mrbconfs with following ways:
  changes `struct` layout and cause memory access error when C and other language(e.g., C++) is mixed.
 
 ## stdio setting.
+
 `MRB_NO_STDIO`
+
 * When defined `<stdio.h>` functions won't be used.
 * Some features will be disabled when this is enabled:
   * `mrb_irep` load/dump from/to file.
@@ -19,80 +23,97 @@ You can use mrbconfs with following ways:
   * Printing features in **src/print.c**.
 
 ## Debug macros.
+
 `MRB_USE_DEBUG_HOOK`
+
 * When defined code fetch hook and debug OP hook will be enabled.
 * When using any of the hook set function pointer `code_fetch_hook` and/or `debug_op_hook` of `mrb_state`.
 * Fetch hook will be called before any OP.
 * Debug OP hook will be called when dispatching `OP_DEBUG`.
 
 `MRB_DEBUG`
+
 * When defined `mrb_assert*` macro will be defined with macros from `<assert.h>`.
 * Could be enabled via `enable_debug` method of `MRuby::Build`.
 
 ## Stack configuration
 
 `MRB_STACK_EXTEND_DOUBLING`
+
 * If defined doubles the stack size when extending it.
 * Else extends stack with `MRB_STACK_GROWTH`.
 
 `MRB_STACK_GROWTH`
+
 * Default value is `128`.
 * Used in stack extending.
 * Ignored when `MRB_STACK_EXTEND_DOUBLING` is defined.
 
 `MRB_STACK_MAX`
+
 * Default value is `0x40000 - MRB_STACK_GROWTH`.
 * Raises `RuntimeError` when stack size exceeds this value.
 
 ## Primitive type configuration.
 
 `MRB_USE_FLOAT32`
+
 * When defined single precision floating point type(C type `float`) is used as `mrb_float`.
 * Otherwise double precision floating point type(C type `double`) is used as `mrb_float`.
 
 `MRB_NO_FLOAT`
+
 * When defined removes floating point numbers from mruby.
 * It makes mruby easier to handle in "Microcontroller without FPU" and "Kernel Space".
 
 `MRB_INT32`
+
 * When defined, or `MRB_INT64` are not defined on 32-bit CPU mode, `mrb_int` will be defined as `int32_t`.
 * Conflicts with `MRB_INT64`.
 
 `MRB_INT64`
+
 * When defined, or `MRB_INT32` are not defined on 64-bit CPU mode, `mrb_int` will be defined as `int64_t`.
 * Conflicts with `MRB_INT32`.
 
 ## Garbage collector configuration.
 
 `MRB_GC_STRESS`
+
 * When defined full GC is emitted per each `RBasic` allocation.
 * Mainly used in memory manager debugging.
 
 `MRB_GC_TURN_OFF_GENERATIONAL`
+
 * When defined turns generational GC by default.
 
 `MRB_GC_FIXED_ARENA`
+
 * When defined used fixed size GC arena.
 * Raises `RuntimeError` when this is defined and GC arena size exceeds `MRB_GC_ARENA_SIZE`.
 * Useful tracking unnecessary mruby object allocation.
 
 `MRB_GC_ARENA_SIZE`
+
 * Default value is `100`.
 * Ignored when `MRB_GC_FIXED_ARENA` isn't defined.
 * Defines fixed GC arena size.
 
 `MRB_HEAP_PAGE_SIZE`
+
 * Defines value is `1024`.
 * Specifies number of `RBasic` per each heap page.
 
 ## Memory pool configuration.
 
 `POOL_ALIGNMENT`
+
 * Default value is `4`.
 * If you're allocating data types that requires alignment more than default value define the
 largest value of required alignment.
 
 `POOL_PAGE_SIZE`
+
 * Default value is `16000`.
 * Specifies page size of pool page.
 * Smaller the value is increases memory overhead.
@@ -100,32 +121,38 @@ largest value of required alignment.
 ## State atexit configuration.
 
 `MRB_FIXED_STATE_ATEXIT_STACK`
+
 * If defined enables fixed size `mrb_state` atexit stack.
 * Raises `RuntimeError` when `mrb_state_atexit` call count to same `mrb_state` exceeds
 `MRB_FIXED_STATE_ATEXIT_STACK_SIZE`'s value.
 
 `MRB_FIXED_STATE_ATEXIT_STACK_SIZE`
+
 * Default value is `5`.
 * If `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined this macro is ignored.
 
 ## `mrb_value` configuration.
 
 `MRB_ENDIAN_BIG`
+
 * If defined compiles mruby for big endian machines.
 * Used in `MRB_NAN_BOXING`.
 * Some mrbgem use this mrbconf.
 
 `MRB_NAN_BOXING`
+
 * If defined represent `mrb_value` in boxed `double`.
 * Conflicts with `MRB_USE_FLOAT32` and `MRB_NO_FLOAT`.
 
 `MRB_WORD_BOXING`
+
 * If defined represent `mrb_value` as a word.
 * If defined `Float` will be a mruby object with `RBasic`.
 
 ## Reduce heap memory configuration.
 
 `MRB_USE_LINK_TIME_RO_DATA_P`
+
 * Only available on ELF platforms.
 * If you specify the address of a read-only section when creating a symbol or string, that string will be used as it is.
 * Heap memory can be saved.
@@ -133,6 +160,7 @@ largest value of required alignment.
 * It must be `__ehdr_start < data_addr < __init_array_start`.
 
 `MRB_USE_CUSTOM_RO_DATA_P`
+
 * Takes precedence over `MRB_USE_LINK_TIME_RO_DATA_P`.
 * Please try if `MRB_USE_LINK_TIME_RO_DATA_P` is not available.
 * The `mrb_ro_data_p()` function is implemented by the user in an arbitrary file.
@@ -140,33 +168,41 @@ largest value of required alignment.
 * Return `TRUE` if `ptr` is in read-only section, otherwise return `FALSE`.
 
 ## Other configuration.
+
 `MRB_UTF8_STRING`
+
 * Adds UTF-8 encoding support to character-oriented String instance methods.
 * If it isn't defined, they only support the US-ASCII encoding.
 
 `MRB_FUNCALL_ARGC_MAX`
+
 * Default value is `16`.
 * Specifies 4th argument(`argc`) max value of `mrb_funcall`.
 * Raises `ArgumentError` when the `argc` argument is bigger then this value `mrb_funcall`.
 
 `KHASH_DEFAULT_SIZE`
+
 * Default value is `32`.
 * Specifies default size of khash table bucket.
 * Used in `kh_init_ ## name` function.
 
 `MRB_NO_METHOD_CACHE`
+
 * Disable method cache to save memory.
 
 `MRB_METHOD_CACHE_SIZE`
+
 * Default value is `256`.
 * Ignored if `MRB_NO_METHOD_CACHE` is defined.
 * Need to be the power of 2.
 
 `MRB_USE_METHOD_T_STRUCT`
+
 * Use C struct to represent `mrb_method_t`
 * No `MRB_USE_METHOD_T_STRUCT` requires highest 2 bits of function pointers to be zero
 * Define this macro on machines that use higher bits of pointers
 
 `MRB_USE_ALL_SYMBOLS`
+
 * Make it available `Symbol.all_symbols` in `mrbgems/mruby-symbol-ext`
 * Increase heap memory usage.
diff --git a/doc/guides/mrbgems.md b/doc/guides/mrbgems.md
index 7120901ee..3dadf4b93 100644
--- a/doc/guides/mrbgems.md
+++ b/doc/guides/mrbgems.md
@@ -18,13 +18,14 @@ You can also use a relative path to specify a gem.
 conf.gem 'examples/mrbgems/ruby_extension_example'
 ```
 
-In that case, 
+In that case,
 
 * if your build configuration file is in the `build_config` directory, it's
   relative from `MRUBY_ROOT`.
 * otherwise, it is relative from the directory where your build configuration is.
 
 A remote GIT repository location for a GEM is also supported:
+
 ```ruby
 conf.gem :git => 'https://github.com/masuidrive/mrbgems-example.git', :branch => 'master'
 conf.gem :github => 'masuidrive/mrbgems-example', :branch => 'master'
@@ -32,17 +33,20 @@ conf.gem :bitbucket => 'mruby/mrbgems-example', :branch => 'master'
 ```
 
 You can specify the sub directory of the repository with `:path` option:
+
 ```ruby
 conf.gem github: 'mruby/mruby', path: 'mrbgems/mruby-socket'
 ```
 
 To use mrbgem from [mgem-list](https://github.com/mruby/mgem-list) use `:mgem` option:
+
 ```ruby
 conf.gem :mgem => 'mruby-yaml'
 conf.gem :mgem => 'yaml' # 'mruby-' prefix could be omitted
 ```
 
 For specifying commit hash to checkout use `:checksum_hash` option:
+
 ```ruby
 conf.gem mgem: 'mruby-redis', checksum_hash: '3446d19fc4a3f9697b5ddbf2a904f301c42f2f4e'
 ```
@@ -66,6 +70,7 @@ via `config.gem`, but wrapped in an `MRuby::GemBox` object.  GemBoxes are
 loaded into mruby via `config.gembox 'boxname'`.
 
 Below we have created a GemBox containing *mruby-time* and *mrbgems-example*:
+
 ```ruby
 MRuby::GemBox.new do |conf|
   conf.gem "#{root}/mrbgems/mruby-time"
@@ -80,14 +85,17 @@ picked up by mruby.
 To use this example GemBox, we save it as `custom.gembox` inside the *mrbgems*
 directory in mruby, and add the following to your build configuration file inside
 the build block:
+
 ```ruby
 conf.gembox 'custom'
 ```
+
 This will cause the *custom* GemBox to be read in during the build process,
 adding *mruby-time* and *mrbgems-example* to the build.
 
 If you want, you can put GemBox outside of mruby directory. In that case you must
 specify an absolute path like below.
+
 ```ruby
 conf.gembox "#{ENV["HOME"]}/mygemboxes/custom"
 ```
@@ -125,6 +133,7 @@ to compile C and Ruby files. *README.md* is a short description of your GEM.
 
 mrbgems expects a specification file called *mrbgem.rake* inside of your
 GEM directory. A typical GEM specification could look like this for example:
+
 ```ruby
 MRuby::Gem::Specification.new('c_and_ruby_extension_example') do |spec|
   spec.license = 'MIT'
@@ -154,6 +163,7 @@ The `license` and `author` properties are required in every GEM!
 
 In case your GEM is depending on other GEMs please use
 `spec.add_dependency(gem, *requirements[, default_get_info])` like:
+
 ```ruby
 MRuby::Gem::Specification.new('c_and_ruby_extension_example') do |spec|
   spec.license = 'MIT'
@@ -177,6 +187,7 @@ end
 The version requirements and default gem information are optional.
 
 Version requirement supports following operators:
+
 * '=': is equal
 * '!=': is not equal
 * '>': is greater
@@ -197,10 +208,12 @@ When a special version of dependency is required,
 use `MRuby::Build#gem` in the build configuration to override default gem.
 
 If you have conflicting GEMs use the following method:
+
 * `spec.add_conflict(gem, *requirements)`
     * The `requirements` argument is same as in `add_dependency` method.
 
 like following code:
+
 ```ruby
 MRuby::Gem::Specification.new 'some-regexp-binding' do |spec|
   spec.license = 'BSD'
@@ -243,7 +256,6 @@ For example: when B depends to C and A depends to B, A will get include paths ex
 Exported include_paths are automatically appended to GEM local include_paths by rake.
 You can use `spec.export_include_paths` accessor if you want more complex build.
 
-
 ## C Extension
 
 mruby can be extended with C. This is possible by using the C API to
@@ -255,6 +267,7 @@ mrbgems expects that you have implemented a C method called
 `mrb_YOURGEMNAME_gem_init(mrb_state)`. `YOURGEMNAME` will be replaced
 by the name of your GEM. If you call your GEM *c_extension_example*, your
 initialisation method could look like this:
+
 ```C
 void
 mrb_c_extension_example_gem_init(mrb_state* mrb) {
@@ -299,7 +312,6 @@ mruby can be extended with pure Ruby. It is possible to override existing
 classes or add new ones in this way. Put all Ruby files into the *mrblib*
 folder.
 
-
 ### Pre-Conditions
 
 none