title: Python,argparse: how to have nargs=2 with type=str and type=int
body: <p>I spent some times on the argparse documentation, but I'm still struggling with this module for one option in my program:</p>
  
  <pre><code>parser.add_argument("-r", "--rmsd", dest="rmsd", nargs=2,
      help="extract the poses that are close from a ref according RMSD",
      metavar=("ref","rmsd"))
  </code></pre>
  
  <p>I'd like to the first argument to be a string (type <code>str</code>) and mandatory, while the second argument should have type <code>int</code>, and if no value is given have a default one (let's say <code>default=50</code>). I know how to do that when there is only one argument expected, but I have no idea how to proceed when nargs=2... Is that even possible?</p>