Make the backend streaming service modular, aka v2.0.0

[christopher-dG]

I'm personally making the switch from GPM to Spotify, and I'd like to support multiple services. I realize that other projects like ncmpcpp, mopidy, etc. already exist for this, but the idea behind this project has always been to "just work" with little configuration required and I don't think that's really the case anywhere else.

This will probably turn into a fairly massive rewrite, which is probably a good idea because I'm a much less bad programmer than I was earlier this year and this code is spaghetti.

A rename will also be in order...