Add lib parameter for cli_exit

I added the `lib` param for `cli_exit`, so that I don't have to resort to hacks like [this](https://github.com/sampsyo/beets/blob/63d939d1d3d6be6163d23c7dca2ae344299deb25/beetsplug/smartplaylist.py#L82), which keeps smartplaylist's (& mpdupdate's when I get to it) code a little bit neater.