add textlint support for markdown (#1383)

ale_linters/markdown/textlint.vim

@@ -0,0 +1,23 @@
+" Author: tokida https://rouger.info
+" Description: textlint, a proofreading tool (https://textlint.github.io/)
+
+function! ale_linters#markdown#textlint#GetCommand(buffer) abort
+    let l:cmd_path = ale#path#FindNearestFile(a:buffer, '.textlintrc')
+
+    if !empty(l:cmd_path)
+        return 'textlint'
+        \    . ' -c '
+        \    . l:cmd_path
+        \     . ' -f json %t'
+    endif
+
+    return ''
+endfunction
+
+
+call ale#linter#Define('markdown', {
+\   'name': 'textlint',
+\   'executable': 'textlint',
+\   'command_callback': 'ale_linters#markdown#textlint#GetCommand',
+\   'callback': 'ale#handlers#textlint#HandleTextlintOutput',
+\})