In conjunction with issue issue #20, add options to control biome size.

Merge 20 into 21 as they are similar things:

#20

This is being looked at for a future release.

will this be a button where you can switsch between small, normal, large, very large (or the "boring" vanilla method, "large biomes" generator), or will it be a slider?

I don't know. This may be moot after 1.3 comes out as Mojang added biome sizing options.