title: argparse funneling positional arguments into multiple lists
body: <p>I would like to be able to support positional command line arguments that go into different lists based on prior predicates.</p>
  
  <p>For example, a command like:</p>
  
  <pre><code>mycommand one two three
  </code></pre>
  
  <p>would yield args like:</p>
  
  <pre><code>main_dest = ['one','two','three']
  other_dest = []
  </code></pre>
  
  <p>but a command like:</p>
  
  <pre><code>mycommand one --other two three --main four five
  </code></pre>
  
  <p>would yield args like:</p>
  
  <pre><code>main_dest = ['one','four','five']
  other_dest = ['two','three']
  </code></pre>
  
  <p>Conceptually what I'd like is an action that modifies the <code>dest</code> of the positional argument reader.</p>