Format at-channel correctly for slack

See https://api.slack.com/docs/formatting for details

ping @mtibben @bradfeehan 
My reading of it is that you have to use `<!channel>`:
> There is a further special token format which is used to denote various things:
>
> <!foo>
>
> Where foo is a special command. Currently defined commands are !channel, !group and !everyone (group is just a synonym for channel - both can be used in channels and groups). Commands that are not recognised can be output as-is, without the enclosing brackets.