Add maintenance mode

[nathampa0909]

Hello, I think the plugin should have a maintance mode that blocks normal players to enter, and allow only that have permission.

Thanks!

[hamcoremusic]

This is a great idea imo. I already use a plugin that blocks players when I set it to maintenance mode and prevent players from logging in.

I would love for there to be like a toggle for a pre-defined maintenance message (defined in config) that you could trigger with a "/slp maintenance" command.