Biom exception list for vanilla bioms that should persist.

[StoneLabs]

I'm currently writing code to implement #10 and #4 so default biome could still be used, but people will also be able to use these two settings

If #10 is implemented and we have a good method of generating vanilla terrain it shouldn't be too difficult to implement an exception list of biomes that will persist (as I described here). Calculate the default biom as well and if it is in the exception list use it instead. I'll go ahead and open a issue for it.

Originally posted by @StoneLabs in #11 (comment)

[StoneLabs]

This will be very useful later on once .17 support is being added. Allowing us to stick to 2D graphics for generation at first and just add the new underground biomes to the exception list by default.