add hooks for when something is required through container

See https://github.com/iammerrick/Squire.js/issues/9. Add hook to register callbacks that are called when a file is required through the container.

This is super dumb right now. It doesn't tell you anything about what was required or offer a way to clean them out, so if you are reusing the same container for different requires and/or want to clean them out between using the container, you are out of luck. This is enough for our needs though, so I was reluctant to add more complexity if it wasn't needed.

Thoughts?