Add the ability to hide from players with "vanish.see"

I have added the `vanish.hide` permission. This acts as an exception to `vanish.see`. If the viewing player has `vanish.see` but a vanished player has `vanish.hide`, he will not be visible. However, if the viewing player also has `vanish.hide`, then `vanish.see` will operate as normal.

This is useful in the case where you wish to have privileged players with `vanish.vanish` and `vanish.see`, but wish to have another set of players capable of safely watching the first set.