add powerline separator for warning section

autoload/airline/themes.vim

@@ -44,3 +44,14 @@ function! airline#themes#get_highlight2(fg, bg, ...)
   let bg = s:get_syn(a:bg[0], a:bg[1])
   return s:get_array(fg, bg, a:000)
 endfunction
+
+function! airline#themes#exec_highlight_separator(from, to)
+  let l:from = airline#themes#get_highlight(a:from)
+  let l:to = airline#themes#get_highlight(a:to)
+  let group = a:from.'_to_'.a:to
+  exec printf('highlight %s guifg=%s guibg=%s ctermfg=%s ctermbg=%s',
+        \ group,
+        \ l:to[1], l:from[1],
+        \ l:to[3], l:from[3])
+  return group
+endfunction