Added proper slope support, fixed bug with one way platforms.
Added initial support for loading and saving levels. To load a level, provide
its name as a command line arg. To save a level, press 's' in editor mode to
save to "default_level".

Tried to implement jumping down from one way platforms, but right now its
limited by my shoddy collision resolution - moving one pixel into the platform
doesn't work, because the collision resolution detects you're in a platform
and moves you right back up. Current implementation teleports you down a whole
tile as a workaround.