New command for cross-posting

[michjnich]

I see there's already #253 for auto-detection of cross-posting. I'm wondering about a simpler option - just adding a /xpost command to bring up a warning not to do it, much like we have /snippet or /justask. It might be nicer having sir botalot do it, than individuals coming across as "policing" the channel - especially when it's newcomers to the channel who may feel put-upon.