Notifying transitioned component that transition has started (instead of ComponentWillUnmount)

[doron-au10tix]

Hi,

I've added this great package to an existing project where "componentWillUnmount" was used to clean all kinds of resources / listeners.
Problem is, now, in my flow, an action occurs, which navigates the user outside of the Component, but a re-render also happens. Usually, without the animation, the component would immediately run "componentWillUnmount" and release all the need for re-renders..
But, now, during the animation bad / unexpcted things happen.

I suggest to add some hook / callback to each such "transitioned" component (e.g. "transitionStarted") to allow it to cleanup sooner, instead of using componentWillUnmount.