Add API for plugins to register custom placeholders

[Prof-Bloodstone]

We need API for plugins to hook into to register custom placeholders. One idea could be AFK plugin adding [AFK] next to player name. Or doing more complex replacements that are not supported by BAT.

[powercasgamer]

This could be implemented?

[Prof-Bloodstone]

This could be implemented?

That also looks interesting. I definitely want to implement PAPI once Velocity version drops, but I don't think it does hurt to have support for few different placeholder providers.