Animation

[MattMcAdams]

I haven't decided if I'm going to include animation styles / classes yet. If I decide to, it'll be on the basis of wanting to have a collection of simple transitions and animations that don't require a library or additional overhead, all wrapped up behind prefers-reduced-motion. This might also get into the territory of having JS scrollspy type thing to kick off the animations when they get on screen, and locking the animations behind javascript being enabled as well.