This looks great - the Slack app is a massive PITA because it's so slow. The only thing stopping me junking Franz for this is that Franz has whatsapp (which you probably can't integrate into Eul anyway because it has to be the web client).
WhatsApp is very tricky, but not impossible.
I thought it had to be via WhatsApp web, no?
Well there is https://github.com/tgalal/yowsup/
but i would not recommend using it