Add pandoc as a markdown fixer (#3641)

Utilize pandoc to fix markdown files, currently set to Github-Flavored
Markdown, but that can be changed by setting,
ale_markdown_pandoc_options.

doc/ale-markdown.txt

@@ -33,6 +33,25 @@ g:ale_markdown_mdl_options                         *g:ale_markdown_mdl_options*
   This variable can be set to pass additional options to mdl.
 
 
+===============================================================================
+pandoc                                                    *ale-markdown-pandoc*
+
+g:ale_markdown_pandoc_executable             *g:ale_markdown_pandoc_executable*
+                                             *b:ale_markdown_pandoc_executable*
+  Type: |String|
+  Default: `'pandoc'`
+
+  This variable can be set to specify where to find the pandoc executable
+
+
+g:ale_markdown_pandoc_options                   *g:ale_markdown_pandoc_options*
+                                                *b:ale_markdown_pandoc_options*
+  Type: |String|
+  Default: `'-f gfm -t gfm -s -'`
+
+  This variable can be set to change the default options passed to pandoc
+
+
 ===============================================================================
 prettier                                                *ale-markdown-prettier*
 

doc/ale-supported-languages-and-tools.txt

@@ -301,6 +301,7 @@ Notes:
   * `languagetool`!!
   * `markdownlint`!!
   * `mdl`
+  * `pandoc`
   * `prettier`
   * `proselint`
   * `redpen`

doc/ale.txt

@@ -2847,6 +2847,7 @@ documented in additional help files.
   markdown................................|ale-markdown-options|
     markdownlint..........................|ale-markdown-markdownlint|
     mdl...................................|ale-markdown-mdl|
+    pandoc................................|ale-markdown-pandoc|
     prettier..............................|ale-markdown-prettier|
     remark-lint...........................|ale-markdown-remark-lint|
     textlint..............................|ale-markdown-textlint|