Add linters for Elixir (#113)

* Add Credo linter for Elixir

* Add requested changes

TODO: check if all message types are covered in `if` chain.

* Add information about Credo linter to README

* Add information about Credo linter to doc

ale_linters/elixir/credo.vim

@@ -0,0 +1,51 @@
+" Author: hauleth - https://github.com/haulethe
+
+if exists('g:loaded_ale_linters_elixir_credo')
+  finish
+endif
+
+let g:loaded_ale_linters_elixir_credo = 1
+
+function! ale_linters#elixir#credo#Handle(buffer, lines)
+  " Matches patterns line the following:
+  "
+  " stdin:19: F: Pipe chain should start with a raw value.
+  let l:pattern = '\v^stdin:(\d+):?(\d+)?: (.): (.+)$'
+  let l:output = []
+
+  for l:line in a:lines
+    let l:match = matchlist(l:line, l:pattern)
+
+    if len(l:match) == 0
+        continue
+    endif
+
+    let l:type = l:match[3]
+    let l:text = l:match[4]
+
+    if l:type ==# 'C'
+      let l:type = 'E'
+    elseif l:type ==# 'R'
+      let l:type = 'W'
+    endif
+
+    " vcol is Needed to indicate that the column is a character.
+    call add(l:output, {
+    \   'bufnr': a:buffer,
+    \   'lnum': l:match[1] + 0,
+    \   'vcol': 0,
+    \   'col': l:match[2] + 0,
+    \   'type': l:type,
+    \   'text': l:text,
+    \   'nr': -1,
+    \})
+  endfor
+
+  return l:output
+endfunction
+
+call ale#linter#Define('elixir', {
+      \ 'name': 'credo',
+      \ 'executable': 'mix',
+      \ 'command': 'mix credo suggest --format=flycheck --read-from-stdin',
+      \ 'callback': 'ale_linters#elixir#credo#Handle' })