title: Prism EventAggregator Subscribe to delegate that references itself to Unsubscribe
body: <p>Is it possible to do something like this:</p>
  <pre><code>EventHandler handler = null;
  handler = (s, args) =&gt;
  {
      DoStuff();
      something.SomeEvent -= handler;
  };
  something.SomeEvent += handler;
  </code></pre>
  <p>with Prism's EventAggregator?
  ie</p>
  <pre><code>Action subscriber = null;
  subscriber = () =&gt;
  {
      DoStuff();
      EventAggregator.GetEvent&lt;SomeEvent&gt;().Unsubscribe(subscriber);
  };
  EventAggregator.GetEvent&lt;SomeEvent&gt;().Subscribe(subscriber);
  </code></pre>