Add a way to stop debouncer

[floatdrop]

Now there is no way to stop running debouncer. With current API it is not trivial to add Stop/Cancel function, because we returning raw function, that is very handy to call.

I think new Option like WithContext could suffice for canceling, but this leads to replacing AfterFunc for select on Done channel and Timer.C channel.

Another thoughts:

• if we cancelling debouncer - should be call debounced function or it should not be called?
• do we even need cancelation (if debounce usually set to small durations)?