Fix numbering of "bugs" test files.
Greatly simplify color shortening.

CSS types now know whether they're expected to be rendered compressed or
not, and so can choose the most appropriate representation.

Colors in particular now try each of several possible renderings and
choose the shortest.  (short_colors is still a separate setting, though
compress implies it.)

This removes the need for a full-document regex to shuffle colors
around, solving several problems related to color names that aren't
actually colors.  It also kills off several regexes and dicts and brings
us closer to Ruby parity.
Remove post_process; now handled by the type system.