Spawning condition customisation

Currently, mob spawning conditions are limited to "Monster" (nighttime spawn), "Creature" (daytime spawn on grass), "Ambient" (spawn anywhere), and "WaterCreature" (in water). You should be able to specify the time of day and what block the mob spawns in or on.

Plant spawning is also limited to certain block types, so it should be customisable to different blocks like custom blocks.