Switch to Sass

I’m working through adding some Sass functionality to Animate.css and using it as the primary language going forward.

We’ll use this PR to discuss development, potential features, and track progress.

Wishlist item(s) I’d like to explore before merging to master:

[ ] Config options for transformations[^1]

[^1]: In other words, adding options for relative vs. absolute transformations (relative to the transformed element vs. fixed, absolute transformation values), and adding some sort of scale for absolute transformations (small, medium, large, extra large, etc.)