Scriptable Beets

While it's true that beets can be extended with modules and plugins, sometimes it's hard to match the flexibility and ease offered by reasonably predictable output and a shell script. This was kicked off by this thread: https://groups.google.com/d/msg/beets-users/riHlzEDzPhE/kPo1GmrHtKMJ

I propose adding a new option to the beet command line (-s, --script, maybe?) that will modify beet's behaviour in three major ways:
- Single threaded execution. It may even be smart to make beets only operate on a single argument or path at a time.
- Verbose output that is easily ingested by other command line tools (grep, sed, etc).
- Exit codes changed up so that testing the exit code is a reliable indicator of whether the script succeeded.

There are a couple of behavioural considerations that I don't have an answer for: Single threaded but accepts multiple argument vs only working on a single input at a time - this might have an impact on how you want to handle stdout. Speaking of output, line buffered output vs buffering the import or the whole script run - need to consider how you'll keep the data in stdout paired with the command being run.

My particular use case involved running a beet import on each new release added to a directory. I only ever have one input argument and thus always know what my output will be concerning. So I don't have a strong opinion about how many arguments --script mode accepts or how the output is buffered. But anyone running bulk operations across large libraries might appreciate a considered solution.