Cleanup of old routes

Awesome stuff @ericvicenti. Super excited to see where this goes.

What are your thoughts around cleaning up old routes? For instance...if you go through like, 10 nested routes...is there something we could include here that would unmount old components? If not unmounting...is there something we might be able to do in React Native land that is like "hey, this view isn't on the screen, can we cleanup after ourselves"? I'm not familiar off the top of my head how UINavigationController handles it behind the scenes...but I know it does something. Perhaps we could follow in it's footsteps, in some way.