Players have requested more flexible options for cave/cavern frequency.

Specifically, from the Discord in regards to caverns:
"Nick is there a way for you to put another chance setting between rare and normal?
My sweet spot for "Floored Caverns" is probably somewhere in that range. Rare has to few and normal has slightly to many"

This makes sense, as the gap in threshold between Rare and Normal for caverns is probably larger than it should be.

Possibilities:

• Add a few extra enum options for frequency
• Add an Additional Settings submenu where the user can control the threshold directly. Perhaps this is only functional when the user sets the frequency enum to something like Custom

Note that this could also be applied to cave/cavern size options as well.