It would be nice if subscribe allowed to define own onMessageArrived handler for subscribed topic.

I'm building a web client to **node-red** where I'm running **mosca MQTT broker**.

I'd like to be able to `subscribe` in the UI client to various topics and process subscribed messages locally within each UI module rather than worry about dispatching them from a shared `mqtt.onMessageArrived` handler.
Perhaps adding an `onMessageArrived` option to the `subscribe` call would make sense for this?
Internally, Paho.MQTT would route incoming messages to that handler if the topic matches subscription filter.

Or, subscribe could return a mediator object that would have its own `onMessageArrived `handler as well as publish method, which would automatically set the topic based on the subscription? However, this may be an overkill and a problem if subscription uses wildcards.