Add options to disable Spigot's /restart and /tps commands

In bukkit.yml, you can now turn off Spigot's registration
of the /restart and /tps commands, with the restart-command
and tps-command settings, respectively.

Useful since Bukkit commands override vanilla commands (for
plugin support), but you might want to use a Forge mod (like
@nallar's TickThreading once it has full MCPC+ support) to
provide the /tps command instead of using Spigot's.