Allow for custom cancellation interfaces.

[Draycia]

In 3.0.0 of Event you were able to override the eventCancelled method to supply your own cancellation logic.

In 5.0.0 with the new EventBus design, this is no longer possible.

One potential idea is to have an EventBus.create overload that accepts a Function2<E, EventSubscriber<? super E, Boolean> for that.