title: Indicate operator-pending mode in vim-airline
body: <p>I am using airline with basic settings. 
  Is there a way to indicate operator-pending mode in the airline status line? In regular, vim default statusline (<code>:set statusline</code>) there is always displayed first letter of a command, whenever vim enters operator-pending mode. I'll be absolutely sufficient for me, but I don't know how should I modify this airline plugin. </p>