title: How to refresh in NERDTree plugin
body: <p>When I open a file in vim with (Directory A in) <code>NERDTree</code>, it works well.</p>
  
  <p>But if I open one more file in another directory (Directory B), it doesn't  refresh to show the contents of directory B (While it still shows directory A).</p>
  
  <p>Can <code>NERDTree</code> automatically refresh by itself?</p>