main: Make :AirlineRefresh! skip recrecrating highlighting groups

When `g:skip_empty_sections` is set, `:AirlineRefresh` can be called
very often and cause slow down, because it forcefully re-creates the
highlighting groups. This is mostly not needed for the redraw to happen,
therefore, add the `<bang>` attribute to the command, making it skip to
re-create the highlighting groups and have all extensions that rely on a
forced update use the `!` form.

Should be much fast then.

fixes #1908

autoload/airline/extensions/whitespace.vim

@@ -165,7 +165,7 @@ function! s:ws_refresh()
   endif
   unlet! b:airline_whitespace_check
   if get(g:, 'airline_skip_empty_sections', 0)
-    exe ':AirlineRefresh'
+    exe ':AirlineRefresh!'
   endif
   let b:airline_ws_changedtick = b:changedtick
 endfunction