Have treasure hunts optionally use separate th_tier%d loot tables (feature)

[sshipway]

This is harder than it sounds as location.py generates chests via dungeon.add_chest, which in turn calls loottable.rollLoot. Overriding would require an override of add_chest in treasure_hunt.py, plus some way to tell rollLoot which set of tiers to use (maybe a prefix parameter?) The treasure hunt tiers should fall back to using the default dungeon tiers if they are not explicitly defined.