Request: better playlist portability

[0chroma]

I like that gonic stores everything as files, but the playlist files it writes use absolute paths, which might change between machines.

It'd be nice if you could specify a custom prefix. For example, I have the following directory structure:

Music/
├── Playlists/
│   └── 1
│       └── playlist1.m3u8
├── Podcasts/
└── Artist 1
       └── Album 1
           ├── 01.song.flac
           └── folder.jpg

It would be nice if I could set the prefix to be ../../ in this case so that the playlists would work even if the absolute path of Music/ changed. The default prefix could just be GONIC_MUSIC_PATH in order to stay consistent with existing behavior.