Should be separate css class added when state transitions to 'entered'

[rmacqueen]

For CSSTransitions, right now the library adds the class example-enter and then, in the next tick, example-enter-active. However it doesn't then add any additional class when the transition completes. What this means is that the element being transitioned has the same CSS in the full entered state as it does in the full exited state, which doesn't really make sense. For example, if fading in a component, I want the full exited state to have opacity 0, and the full entered state to have opacity 1. There is no way to do this using the CSSTransition component (though you can achieve it using the lower-level Transition component).
Ideally it would add something like example-entered once in the entered state.

[taion]

This was resolved in #269.