Support a `Stop()` method on handlers

[phemmer]

Some handlers might spin up goroutines, and so we should support a way to shut the handler down when it has been removed.
Keep the Handler interface minimal (Event() only), but do a type assertion on it to see if it has Stop(), and call it.

Note that this might be a little less straightforward, as we want to support removing a handler, and then adding that same handler back (such as if we wanted to interpose a filter).
But some way we need to call Stop() once the handler is no longer used.