Add support for permissions while players are offline

[baailey]

If you have openinv.exempt your inventory will be protected, however, that is not the case if the player is offline when you try to view their inventory

[Jikoo]

Bukkit does not provide a system for this, and logically permissions plugins don't load data for all players. Will need to add a Vault integration.

This has been on the backburner for a while, but it's good to have it documented.