Add best-so-far implementation of moving platforms.

This is a sloppy commit. I wanted to get this committed before I forgot
and accidentally screwed something up. This is as good as it's ever
been.

Known problems:
- First vertical collision with platform is a bit jarring as it balances
out the position
- Slight visual pop-through when jumping into it from underneath, but
this is probably fixable by adjusting the sprite rather than code
- HACKY FIXES TO EVERY PROBLEM