Add -playlists-relative option

[0chroma]

Fixes #529

Improves portability between machines by optionally making file paths relative to the playlist

feedback welcome! it's my first time writing go

[sentriz]

hm I wonder do we need the config option at all? can we just allow relative paths? joining on the basedir if the playlist line doesn't look absolute with no leading slash

[0chroma]

the option mostly determines if the playlist is written with absolute or relative paths, I modified the reader so it does exactly what you described regardless of config option

[0chroma]

If you'd like I can remove the config check during writes from the PR, but I think there's a lot of situations where absolute paths would still make sense (ie having playlists and music on separate volumes)

[0chroma]

heya, just checking in. Let me know if you'd like this merged in!