Option to restrict `/mc /playerlist /whitelist` to the chat channel.

[irunatbullets]

On a discord server running multiple Minecraft servers and blockbots, it would be great to have an option to restrict all of the bot commands to the chat channel specified in the config.

There's no way, for example, to prevent people from using /whitelist in random channels.

And if we have two blockbots running, it might be a bit confusing if we ever need to /mc stop a server.

One drawback is that it would be awesome to be able to /mc ban across all servers with a command run only once, but we're used to doing it once per channel anyway.