Add mpris-service to handle mediakeys

[TommyAmberson]

I wanted to be able to use my mediakeys to control pianobar. Mostly I wanted it to pause when I tapped my earbuds.

Tested on ubuntu:

• Shows the current song and controls in the notification bar.
• Media keys (pauseplay, pause, play, next, quit) work

[TommyAmberson]

further reading: https://www.npmjs.com/package/mpris-service