Add ability to enable/disable plugin in specific worlds.

Commands:

* `/mychunk world enable` - Enables current world
* `/mychunk world disable` - Disables current world
* `/mychunk world enable <world>` - Enables named world
* `/mychunk world disable <world>` - Disables named world
* `/mychunk world enable all` - Enables all worlds currently loaded on the server
* `/mychunk world disable all` - Disables all worlds (clears the list)

Permissions:

* `mychunk.commands.world.*` - All of the below
* `mychunk.commands.world.enable` - Enable current or named world
* `mychunk.commands.world.disable` - Disable current or named world
* `mychunk.commands.world.enable.all` - Enable all worlds
* `mychunk.commands.world.disable.all` - Disable all worlds

Enabled world list is kept on a Set<String> static member "worlds".

All events listened for are only handled if the world the event occurred in is in the enabled worlds list.