room and room extension rendering
refactor PNG export to PngExporter
refactor field serialization in PngExporter to allow customization
remove Feature class, move Drawing to Dungeon
refactor fill_rect with for_rect
rename render_extension -> render_annex
Room#render will always render in the same place
move features spec to spec/pulp_dungeon_generator
refactor Dungeon#find with find_if
a few more appropriate names