title: Add widget to extension
body: <p>Is it possible to add/use a widget to/for an extension?</p>
  
  <pre><code>def group(cls, admin_cls):
  
      cls.add_to_class('group',
          models.ManyToManyField(
              Group,
              widget=?????
            ))
  </code></pre>
  
  <p>I want to use Django's FilteredSelectMultiple for the many to many field.</p>