Animation persistence

[jorgecasar]

This implements animations persistence. It's very useful for pairs animations such as fade-in/fade-out, because you don't have to manage the final state based on a local variable to know what's the state of the component. And you can avoid the use of a listener to wait for the animation finished.

[addyosmani]

Thanks for your patch to the project, @jorgecasar. After reviewing your suggestions for animation persistence, I'm of the opinion that we're not quite ready for this in master. The listener approach for waiting on the animation to end has served us well so far. We haven't had demand for supporting this type of feature (yet) so we're going to skip this PR for now. If folks get back to us about wanting this in at some point we'll consider reopening this PR.

Cheers once again for trying to help!