Added worldgen and keeploaded world options to bukkit.yml. Fixes #1081

Fixes #1005, 1052, #1053, #1080, #1095

* Each mod world generator will now be automatically added to bukkit.yml
as they are registered by the mod. To disable worldgen from a specific
generator, set it to false. Note: Many mods rely on their world generators
so disable with caution.
* Environment keeploaded flags are now added to bukkit.yml as they are
registered. To prevent an environment from being unloaded, set it to
true. For example, if you want to prevent all dimensions using "the_end"
environment from unloading set "keeploaded-environment-the_end" to true in
bukkit.yml. Note: This has no effect on dimensions being loaded at
startup. It simply controls whether or not a dimension can be unloaded.