Add plist

Adds the launchd plist from mopidy/mopidy#887 to the mopidy formula.

After installing mopidy, the user will see instructions for daemonizing it with launchd:
```
==> Caveats
To have launchd start mopidy at login:
    ln -sfv /usr/local/opt/mopidy/*.plist ~/Library/LaunchAgents
Then to load mopidy now:
    launchctl load ~/Library/LaunchAgents/homebrew.mxcl.mopidy.plist
```