Make Slack token configurable

Current implementation line [located here](https://github.com/reaction-eng/restlib/blob/2f7efe1912e35f2df85e23dfd84e03e57f270fa4/Notification/NotifierSlack.go#L26).

Slack disabled this token automatically long, long ago thanks to the original author not making the token configurable (and secret) in the first place.