Add JSON output option

While this could be managed using the format option, I figure that could easily become unwieldy, and would be better suited as an internal function.

My goal is to use beet spawned as a sub-process and then doing actions based on the results. Initially, I'm thinking about using a search query and parsing the results for a very-smart-playlist (adding some randomization and conditions not natively supported, nor do I think they would be useful outside of my narrow usecase).

I also imagine that the results could be used in a lot of other situations, similar to the [ecosystem of tools surrounding TaskWarrior](http://taskwarrior.org/projects/taskwarrior/wiki/ExternalScripts)

Not all of the commands need JSON output, I imagine that the following would suit most uses:
- info
- search
- list
- stats (though it's already in a fairly machine friendly format)
- version (same as stats)