title: Python argparse type and choice restrictions with nargs > 1
body: <p>The title pretty much says it all. If I have nargs greater than 1, is there any way I can set restrictions (such as choice/type) on the individual args parsed?</p>
  
  <p>This is some example code:</p>
  
  <pre><code>parser = argparse.ArgumentParser()
  parser.add_argument('-c', '--credits', nargs=2,
      help='number of credits required for a subject')
  </code></pre>
  
  <p>For the -c argument I need to specify a subject and how many credits are required. The subject should be limited to a predefined list of subjects, and the number of credits required should be a float.</p>
  
  <p>I could probably do this with a subparser, but as it is this is already part of a sub-command so I don't really want things to get any more complicated.</p>