OpGuard isn't usable on some hosts that use custom panels

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Some hosts have panels that use rcon instead of direct console access.
(Which is absolutely strange and terrible, but they do indeed exist)

Users on these hosts cannot access /opguard from console.

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RCON isn't able to execute /opguard because it isn't registered as a command. Instead, OpGuard listens to command-related events in CommandInterceptor.java, which aren't called when using RCON.

Proposed solution:

• Add a config option for registering /opguard into Bukkit's CommandMap.