ignore alternative channels; support private groups

[kballenegger]

This is pretty much my first time writing real Go code, so it's probably not pretty.

This changes two things:

• Only accepts commands from specified channel.
• Allows musicbot to work with private groups.

[kballenegger]

If this change make sense, I'll clean up the log statements / typos, etc. :+1: ?

[sosedoff]

Sure, go ahead