title: argparse set default to multiple args
body: <p>I want use argparse to specify a option that take three integer values: like specifying a range: start, end, step. </p>
  
  <pre><code>parser.add_argument('--range', nargs=3, metavar=('start', 'end', 'step'), 
               type=int, help="specify a range')
  </code></pre>
  
  <p>However, it is not clear to me how to provide default values to all three values. Do I need to define it as string and parse it myself? </p>