Add an interface and a way to associate dimlets with that interface to get custom structures at spawn position in an rftools dimension. Useful in combination with mods like Recurrent Complex and similar.

With #96, this would be a very self-explanatory add: The Dimlet would be prompted to generate the spawn structure with DimletStack, World, x, y, z, where x, y, z is the position the player will spawn at. Recurrent Complex would simply add and register a Dimlet instance which implements this functionality using structures stored in the DimletStack NBT by String ID / structure data.