[slack] Add support for notifying multiple channels
[slack] Add support for notifying multiple channels

Fixes #19

chrippa/blues,adisbladis/blues,5monkeys/blues,chrippa/blues,5monkeys/blues,chrippa/blues,Sportamore/blues,Sportamore/blues,Sportamore/blues,adisbladis/blues,5monkeys/blues,adisbladis/blues