Changing some spectator things

[emielderckx]

Idea:

• Config or permission to stop changing the tab when a player is in spectator mode.
• A config or permission to change if spectators can see other spectators

Describe alternatives you've considered.

I know that a some plugins offer these features, I use my self NoSpectTab. But I still think this is a feature that fits good with Purpur and that a lot of users will like it.