dillon1000/react

[ReactTransitionGroup] Add onTransitionEnter and onTransitionLeave

It is valuable to know when the number of children in a TransitionGroup is going
to increase or decrease, since we might want to apply extra animations.
For instance, when used with overflow:auto, we might want to apply different css
based on it overflowing or not - to do this we need to calculate this after new
nodes has entered and after nodes has been removed.