title: Argparse optional positional arguments?
body: <p>I have a script which is meant to be used like this:
  <code>usage: installer.py dir [-h] [-v]</code></p>
  
  <p><code>dir</code> is a positional argument which is defined like this:</p>
  
  <pre><code>parser.add_argument('dir', default=os.getcwd())
  </code></pre>
  
  <p>I want the <code>dir</code> to be optional: when it's not specified it should just be <code>cwd</code>.</p>
  
  <p>Unfortunately when I don't specify the <code>dir</code> argument, I get <code>Error: Too few arguments</code>.</p>