Add configurable stop mode

[GitGhillie]

Partial solution to #72 as it would now be possible to configure a new sound with stop_mode = None. Then you could despawn the entity immediately and the sound would play until the end.

Maybe a better solution would be to create a new enum instead of the Option<StopMode>. That way we can give it a more appropriate name and avoid the necessity for users to import libfmod::StopMode. LMK

[GitGhillie]

Another, more simple solution would be to just remove self.event_instance.stop(Immediate).unwrap(); from the Drop implementation, and then leave it up to the user to call stop() or not before despawning an entity.