Callbacks for subscribe/unsubscribe methods

[JustBlackBird]

It could be useful to have callbacks for subscribe/unsubscribe methods. In this case a client could manage subscriptions easily.

I could try to make a PR with the changes if the maintainer is interested in it.

[gdaws]

If you want to make a pull request then go ahead. For it to be accepted though subscribe and unsubscribe must have the same interface that the send function has for passing an onReceipt callback through the options parameter.