Subscrition callback filters support

[mauroservienti]

Connects to #239
Fix #239

This PR introduce support for subscriptions filters allowing something like the following:

broker.Subscribe<MyMessage>( 
   subscriber: this,
   callbackFilter: (s, msg) => { /* filter */ return true; },
   callback: (s, msg) =>{ /* logic */ } 
);

[micdenny]

I think is good to go!!