[matrix] notification provider

[b4nst]

Just throwing that here, but it would be cool to add a [matrix] providers to send notification on a room and/or person (like it is for Telegram)

[RickyGrassmuck]

i'll have to do a bit more digging but the API documentation I was able to find looks very incomplete and the little I was able to gather while looking into it made it look far from trivial.