Mattermost has come up a couple of times recently. Along with Matrix, it's probably our top requested platform. Right now we're focused on improving the core platform, but the nice thing is when we add new platforms all the skills written for other platforms should work on day one. I added your request to our internal tracker for Mattermost and will try to reach out once we add it (your email isn't in your profile, so if you want to email me at paul@ab.bot I can add you to the list).