[Core] Providing completion for very basic handlers

[kartikthapar]

In some cases, I have felt the need to override some of the completion blocks.
This requires me to provide the completion block as part of the helper function itself.

I also wanted to provide another parameter for unwindToViewController but given how its being used in the current framework and possible external usages, its going to break every single flow.

[lkzhao]

You are definitely correct, these completion blocks should be added. And also into unwindToViewController as well. At the moment unwindToViewController is more of a hack rather that a robust API and we should put some thoughts into how we should change it.

Anyway, thanks a lot for the contribution!