Persist events that not yet have a handler

There's a risk that a event is emitted before its' handler has been registered. You might say that such code is bad design, but I'd like to be lazy ;) Also, given that code splitting is being more and more adopted, I think this scenario is very real.

Would it be possible to persist events that don't have a handler (yet)? Whenever `.on(...)` is run, mitt would then check the persisted events in order to find unprocessed ones, and then handle those events immediately.

I think the footprint for this change can be quite small, but I can't speak for performance. Maybe we can opt-in to the behaviour.