Add a linter for alex

https://github.com/wooorm/alex

Enabled for text-like file formats and documented in README and doc.

autoload/ale/handlers/alex.vim

@@ -0,0 +1,22 @@
+" Author: Johannes Wienke <languitar@semipol.de>
+" Description: Error handling for errors in alex output format
+
+function! ale#handlers#alex#Handle(buffer, lines) abort
+    " Example output:
+    "       6:256-6:262  warning  Be careful with “killed”, it’s profane in some cases      killed           retext-profanities
+    let l:pattern = '^ *\(\d\+\):\(\d\+\)-\(\d\+\):\(\d\+\) \+warning \+\(.\{-\}\)  \+\(.\{-\}\)  \+\(.\{-\}\)$'
+    let l:output = []
+
+    for l:match in ale#util#GetMatches(a:lines, l:pattern)
+        call add(l:output, {
+        \   'lnum': l:match[1] + 0,
+        \   'col': l:match[2] + 0,
+        \   'end_lnum': l:match[3] + 0,
+        \   'end_col': l:match[4] - 1,
+        \   'text': l:match[5] . ' (' . (l:match[7]) . ')',
+        \   'type': 'W',
+        \})
+    endfor
+
+    return l:output
+endfunction