enable pan back animation for 1side+center setup

I use RNSwipeViewController in an app where I only need center
and left controllers. If I don't setup right controller I don't get
animation when I want to pan back from left to center controller.
I added condition that animation should be skipped only if opposite
side controller is not set and also center controller is
centered in view. I also made canShowLeft / canShowRight
checks more explicit because this way it's easier
to understand the condition.
Merge pull request #2 from linkov/patch-1

enable pan back animation for 1side+center setup