Add support for adding multiple subscriptions to a composite subscription at once

[ZakTaccardi]

Instead of multiple compositeSubscription.add(subscription) calls, it would be nice to be able to do something like compositeSubscription.add(Subscription... subscriptions).

Currently you can only do something like this in the constructor for CompositeSubscription.

[akarnokd]

Hi. What is your use case for it? Does it have to be atomic?

I particularly never needed such a method but PRs are welcome.

[ZakTaccardi]

It would help reduce the verbosity of multiple compositieSubscription.add(subscription) calls.

I'm calling it from the main thread (android) to register for events, so no it would not need to be atomic for my specific use case.

[akarnokd]

#3720

[akarnokd]

Closing via #3720