[ENH] Support mentions

[fraimondo]

I was looking for a replacement for the github slack bot that works with Matrix and I have to say that this maubot plugin is just what I needed! Before trying this, I was using opsdroid. One of the features I was thinking of implementing is to convert github mentions into matrix mentions.

Will this be possible? I can work on it and create a PR. I was thinking that maybe someone will insight on the project could point me in the right direction where to start.