Adding #slack support with the same interface

Lots more stuff can be added to make this optimized for #slack, but for now, just trying to plug things in. @MrPowers have a look. cc @alexaandru @eclosson 

Needs tests! Feel free to keep going with this...
Looks good Daniel :+1:  I looked at the Slack website and it looks promising.

On this line, think you just need to capitalize the "s"