Format at-channel correctly for slack

See https://api.slack.com/docs/formatting for details

ping @mtibben @bradfeehan 
Isn't `U024BE7LH` just a sample ID for “bob” in the docs at https://api.slack.com/docs/formatting ?
And `C024BE7LR` a sample ID for their `#general` channel?
(I could be wrong, the Slack docs are pretty unclear)