0.3.1

Remove yougai chest/crotch dark spots, update README, bump to 0.3.1

The chest shading pixel added for the muscular redesign read as
nipples, and the long transparent gap between the legs (needed to
separate them visually) showed through as a dark strip at the groin
on a dark background. Removed the chest shading and merged the legs
into a single connected column that only splits apart at the feet.

Also updated the README to describe the current skin lineup (dog/cat
/girl/yougai/fish), the sea-creature companion reskin, the real
companion cap (15), and the scripts/gen-sprites.js sprite tool, since
it still described the old white-square-only appearance.