Add support for ALE, the Async Linting Engine

Ale is a plugin that lints text in the background, as you type!

It is new, but under [rapid development](https://github.com/w0rp/ale).

autoload/airline/extensions/ale.vim

@@ -0,0 +1,36 @@
+" MIT License. Copyright (c) 2013-2016 Bjorn Neergaard.
+" vim: et ts=2 sts=2 sw=2
+
+if !exists('g:loaded_ale')
+  finish
+endif
+
+let s:error_symbol = get(g:, 'airline#extensions#ale#error_symbol', 'E:')
+let s:warning_symbol = get(g:, 'airline#extensions#ale#warning_symbol', 'W:')
+
+function! s:count(index)
+  let l:buf = bufnr('%')
+  let l:count = ale#statusline#Count(l:buf)
+  if type(l:count) ==# type(0)
+    let l:count = 0
+  else
+    let l:count = l:count[a:index]
+  endif
+
+  return l:count
+endfunction
+
+function! airline#extensions#ale#get_errors()
+  let l:count = s:count(0)
+  return l:count ? s:error_symbol . l:count : ''
+endfunction
+
+function! airline#extensions#ale#get_warnings()
+  let l:count = s:count(1)
+  return l:count ? s:warning_symbol . l:count : ''
+endfunction
+
+function! airline#extensions#ale#init(ext)
+  call airline#parts#define_function('ale_error_count', 'airline#extensions#ale#get_errors')
+  call airline#parts#define_function('ale_warning_count', 'airline#extensions#ale#get_warnings')
+endfunction