Suggestion: Add ability to use existing channel for starboard

[Kenny2github]

Might be useful in cases where an existing starboard is being migrated to R. Danny

[Rapptz]

Historically I've been against because the bot makes a lot of assumptions about the channel, like what permissions it has and whether messages come from the bot itself etc. The channel is meant to be managed exclusively by the bot, so allowing anyone to pick any pre-existing channel kind of throws a wrench in it.

I'll have to see if it causes any issues first.