Broaden custom setlist creation options

We have a genre_to_playlist function which can convert a given list of genres to a playlist, but why stop there?

With a view to creating a website, we should create a flexible engine that takes a list of parameters to craft more specific playlists. These parameters should include the spotify track audio features listed [here](https://developer.spotify.com/documentation/web-api/reference/#/operations/get-audio-features)

We should also consider track details, for example the year or decade the track was made, the language the song is in etc etc.

Start by reading the existing genre_to_playlist function and the Spotify audio-features reference linked in the issue. Clarify which playlist parameters and track details are required before implementation. Done would mean a defined, flexible playlist-creation engine supporting the agreed parameters, but the issue does not specify acceptance criteria or tests.

Written by the indexing model from the issue text.