title: MEF recursive plugin search
body: <p>Let's say that I have a few applications in a folder (each application has subfolders where plugins can be located):</p>
  
  <ul>
  <li>Clients
  <ul>
  <li>Application A
  <ul>
  <li>...</li>
  </ul></li>
  <li>Application B
  <ul>
  <li>...</li>
  </ul></li>
  <li>Application C
  <ul>
  <li>...</li>
  </ul></li>
  <li>...</li>
  </ul></li>
  </ul>
  
  <p>Some files in these applications have an Export-attribute applied, others don't. Now, I want to be able to load these plugins in some of these applications. Is there a proper way to let MEF search recursively in every subfolder of a specified folder?</p>