title: zsh completion according to a parameter
body: <p>I am a quite a happy zsh user and especially appreciate its completion subsystem.
  Recently I am trying to enhance some completion commands especially those I use often (ex. rm).
  In zsh the following command:</p>
  
  <pre><code>rm &lt;TAB&gt;
  </code></pre>
  
  <p>makes zsh complete with the current directory's files as expected.
  What I am trying to do is make zsh complete the following variant:</p>
  
  <pre><code>rm -r &lt;TAB&gt;
  </code></pre>
  
  <p>with only directories.</p>
  
  <p>Any zsh Guru that knows how to parameter completion so that it takes in consideration the (-r)
  parameter?</p>