Adjust island-level by a static value

[tastybento]

https://github.com/tastybento/bskyblock/issues/69

From @ghacproductions on May 14, 2017 19:21

Hey,

I've checked both the plugin-site on spigotmc.org and the source-code (atleast a quick check) and could not find a (possible) feature:

Adjusting the island-level of a player(his island) by a static value (setting, adding or removing an amount of levels).
This would be very useful for giving whole levels for specific challenges as the challenges could simply execute an "island leveladd" command to give a specifc amount of levels (points). Additionally, other plugins could hook into the system and execute commands to give islandlevel.

If this is an appreciated feature, I'ld happily create a PR for this feature.

Greetings
ghac

Copied from original issue: tastybento/ASkyBlock-Bugs-N-Features#220