Support passing directories to mopidy --config

This makes it possible to pass directories, as well as files, to the --config option. The main use for this would be to pass /etc/mopidy/extensions.d without the caller needing to check which files that exist in this directory.

Although this isn't a strictly a bug fix, I would like to get it into 0.19.4. The reason for this is that I'm making some changes to submit to the arch package to make it more similar to the debian package, and I would like to not have to wait for 0.20.0 for this. Arch uses systemd, so it can't list the config files in /etc/mopidy/extensions.d and pass those to --config. Therefore, this change is necessary.