title: give a hint when press prefix key in tmux
body: <p>When I press the prefix-key <kbd>Ctrl-b</kbd> tmux doesn't give me a hint that it has been pressed.</p>
  
  <p>Sometimes I can not remember whether I have already pressed it or not.</p>
  
  <p>Can I set up a hint like highlight/change color or show some special symbol in the status bar to show me when I have pressed the prefix-key?</p>