title: Python Argparse: Mutually exclusive group where one or the other are required
body: <p>I'm trying to build an input list where I have two options -</p>
  
  <pre><code>domains.add_argument(
      '-d', dest='domain', required=True,
      help=   'Specify a target domain name'
  )
  </code></pre>
  
  <p><strong>domain</strong> - which would take input of a domain. And the same again, but -dL (or <strong>domainlist</strong>). If a domain is parsed then domainslist isn't required, <em>but one of them must be provided.</em></p>
  
  <p>I've tried to do this by adding both of these as required in a mutually exclusive group, however the error I'm presented with states that no objects in a mutually exclusive group can be required.</p>
  
  <p>I could hardcode this check on startup (make neither required and handle myself) but I'm sure argparse can do it, can somebody gfive me a nudge as I haven't found this in the documentation thus far.</p>