Lair Creatures can spawn in world

[Aintnobodyreal]

Since especially on multiplayer servers dragons and other monsters that only spawn on worldgeneration in lairs get scarse over time I propose adding a config option that allows monsters that normaly can't spawn after worldgeneration to have a chance to do so. Basically adding an option (with probability) for mobs like dragons (split by stages), zyklops and alike to spawn at random in the world. This could be further enhanced by also adding biome and dimenson lists (fire dragons in the nether sounds interesting) to determine where they can spawn.