add redpen support

ale_linters/markdown/redpen.vim

@@ -0,0 +1,32 @@
+" Author: rhysd https://rhysd.github.io
+" Description: Redpen, a proofreading tool (http://redpen.cc)
+
+function! ale_linters#markdown#redpen#HandleErrors(buffer, lines) abort
+    " Only one file was passed to redpen. So response array has only one
+    " element.
+    let l:res = json_decode(join(a:lines))[0]
+    let l:errors = []
+    for l:err in l:res.errors
+        if has_key(l:err, 'startPosition')
+            let l:lnum = l:err.startPosition.lineNum
+            let l:col = l:err.startPosition.offset
+        else
+            let l:lnum = l:err.lineNum
+            let l:col = l:err.sentenceStartColumnNum + 1
+        endif
+        call add(l:errors, {
+        \   'lnum': l:lnum,
+        \   'col': l:col,
+        \   'text': l:err.message . ' (' . l:err.validator . ')',
+        \   'type': 'W',
+        \})
+    endfor
+    return l:errors
+endfunction
+
+call ale#linter#Define('markdown', {
+\   'name': 'redpen',
+\   'executable': 'redpen',
+\   'command': 'redpen -r json %t',
+\   'callback': 'ale_linters#markdown#redpen#HandleErrors',
+\})