Explore transition application to specific view controllers

[jverkoey]

Related to #90, it may be nice to able to pick which view controller a transition affects, rather than having transitions always assume what should move.

For example, in a typical transition with a back and fore view controller, we might want to slide the fore view controller and scale the back view controller.

One option (I don't like this):

viewController.transitionController.foreViewControllerTransition = SlideTransition.self
viewController.transitionController.backViewControllerTransition = ScaleTransition.self