Summary:
This follows up the PR https://github.com/facebook/react-native/pull/8306
= Changes
1. Provides the TransitionProps (current and previous) to the function
`configureTransition`.
2. Adds a new member `timing` to `NavigationTransitionSpec`.
= Why
1. Helps people to customize the animation between transitions
2. Helps people to migrate from the deprecated `applyAnimation` prop.
Reviewed By: ericvicenti
Differential Revision: D3470802
fbshipit-source-id: be49becccd53aed7bc57093da1c7dae20153febd