Refactor async functions

- create a new async module
- refactor async functions from branch.vim and po.vim to async.vim
- support nvim async correctly

autoload/airline/util.vim

@@ -86,42 +86,3 @@ else
     return 0
   endfunction
 endif
-
-" Define a wrapper over system() that uses nvim's async job control if
-" available. This way we avoid overwriting v:shell_error, which might
-" potentially disrupt other plugins.
-if has('nvim')
-  function! s:system_job_handler(job_id, data, event) dict
-    if a:event == 'stdout'
-      let self.buf .=  join(a:data)
-    else " on_exit handler
-      if self.buf =~? ('^' . self.cfg['untracked_mark'])
-        let self.cfg.untracked[self.file] = get(g:, 'airline#extensions#branch#notexists', g:airline_symbols.notexists)
-      else
-        let self.cfg.untracked[self.file] = ''
-      endif
-    endif
-  endfunction
-
-  function! airline#util#system(cfg, file)
-    let l:config = {
-    \ 'buf': '',
-    \ 'cfg': a:cfg,
-    \ 'file': a:file,
-    \ 'cwd': fnamemodify(a:file, ':p:h'),
-    \ 'on_stdout': function('s:system_job_handler'),
-    \ 'on_exit': function('s:system_job_handler')
-    \ }
-    let cmd = a:cfg.cmd . shellescape(a:file)
-    let l:id = jobstart(cmd, l:config)
-    if l:id < 1
-      return system(cmd)
-    else
-      return ''
-    endif
-  endfunction
-else
-  function! airline#util#system(cmd)
-    return system(a:cmd)
-  endfunction
-endif