map_editor: red: read tileset gfx filenames from source
use config instead of configuration

this was kind of overzealous
map_editor: more flexible macro handling

makes less assumptions about the structure of a header
map_editor: skip any nonexistent tiles in map rendering
map_editor: create any pngs that don't exist yet
crystal.py: no more relative paths
Merge branch 'yenatch/map-editor' into master

Conflicts:
	pokemontools/crystal.py

Fixes #40.