Support for Overlapping Subscriptions

[ghillert]

This may be a feature request.

In certain situations a component may create multiple Observables whose Subscriptions I would like to add to the busy indicator. Those Subscriptions may be the result of different method calls. Therefore, "overlapping Subscriptions" could be a possibility.

It would be nice if Angular2 Busy could consume a stateful array (or similar) and upon completion remove Subscriptions from that array.