Add mpris support

[alrayyes]

It would be nice to be able to control playback with playerctl. VLC appears to support this out of the box, and mpv with the mpv-mpris plugin installed.

Hoever it appears libmpv, which python-mpv appears to use does not load additional installed mpv plugins, although i could be wrong here. Is there a trivial way to add mpris support?

[davidbosman]

I'd love that, or any workaround that would let me use multimedia controls while Castero plays in the background. I take a lot of notes while listening to some podcast, not having to switch between my text editor and Castero every time I want to pause/resume a podcast would be great :)