Flaming material spills

[red-kangaroo]

Allow flammable liquids and gases to catch fire when spilled, coating the given tile in flames.

Possible usage: broken lanterns would spill burning oil rather than making a small explosion, explosive gas that would be harmless until touched by a flame, sulphur golems would be much more interesting.

Could (should?) also replace current FIRE_GAS, which only deals fire damage and does not directly interact with the fire subsystem.