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.

autoload/ale/fixers/pandoc.vim

@@ -0,0 +1,16 @@
+scriptencoding utf-8
+" Author: Jesse Hathaway <jesse@mbuki-mvuki.org>
+" Description: Fix markdown files with pandoc.
+
+call ale#Set('markdown_pandoc_executable', 'pandoc')
+call ale#Set('markdown_pandoc_options', '-f gfm -t gfm -s -')
+
+function! ale#fixers#pandoc#Fix(buffer) abort
+    let l:executable = ale#Var(a:buffer, 'markdown_pandoc_executable')
+    let l:options = ale#Var(a:buffer, 'markdown_pandoc_options')
+
+    return {
+    \   'command': ale#Escape(l:executable)
+    \       . ' ' . l:options,
+    \}
+endfunction