Provide a mechanism to have specialized dispatchers

[umamaistempo]

Depends on #11

It might be useful to have a way to extend dispatchers. That way we could override their pre-hook to provide better handling on special cases.

Eg: Dispatcher A defines several events, Dispatcher B extends it with an specialized pre-hook that stores on an external service each event (that way with a simple switch you can log certain events)

Eg: Dispatcher A defines several events, Dispatcher B extends it by providing additional event handlers, that way a certain part of the application can only emit Set<[A]> while another part can emit Set<[A, X]> (being X the new events/handlers provided by B)

That also allows complementary behaviours on certain environments (eg: additional logging step on test) etc