Add World:onEmit

[pablomayobre]

This function would allow you to define a callback that would be called before an event is emitted.

This is really useful for debugging event delegation.

This callback would get a reference to the World, so you could emit another event from this callback (A note should be made here that this new event should be ignored by :onEmit so that it doesn't end up in an infinite emit-onEmit-emit-onEmit loop).