Simple permission plugin

[boformer]

There should be a simple reference plugin that provides the permission service.

• Store permissions in hocon config
• Reload command (no ingame permission modification)
• Basic group support?
• Dimension and World context (or combination of both)

[stephan-gh]

We need to have the Permission API implemented in the implementation first because writing such a reference plugin wouldn't make any sense right now, you couldn't test it.

[boformer]

@Minecrell SpongePowered/Sponge@92bee47