title: Override autofac registration with plugin
body: <p>I have an <code>IFoo</code> service implemented by <code>DefaultFoo</code>, and I've registered it as such in my autofac container.</p>
  
  <p>Now I would like to allow for an alternative implementation of <code>IFoo</code> to be implemented in a plugin assembly, which can be dropped in a "plugins" folder. How do I configure autofac to prefer this alternative implementation if it is present?</p>