Make nether island region heights configurable

[addstar]

As discussed in #1172, the nether region height was inadvertently adjusted a while ago. And although it's now fixed in master, I thought it would be good to have this configurable.

Server owners can now set minY and maxY for the island regions created in the nether.

The default values in config (and code) are now:

worldguard:
  nether:
    minY: 6
    minX: 120

[Muspah]

Thanks for your PR! Sounds like a good idea to make this configurable, but I'm not sure if we want the user to directly control the setY() calls via the config option. It might create cases where users put in impossible values. Another thing we need to check is if/how the user can recreate the WorldGuard regions (protectall?) after changing this settings.