Add support for song-mapping with local audio files

Currently, songs are only stored by their metadata and e.g. their Spotify ID. It would be nice, however, to be able to map them to actual audio files if the user has the corresponding song locally on their computer (e.g. by providing drum a folder where music is stored).

This matching should ideally be fuzzy (work with a variety of formats, including MP3 tags or when only title and artist are stored in some combination in the title).

Once mapped, the local audio file path should be stored in the track, similar to how Spotify and Apple Music IDs are stored currently.

Local audio file mappings would also let us properly handle services that rely on local paths such as
- https://github.com/fwcd/drum/issues/21
- https://github.com/fwcd/drum/issues/4