Compute sparse distance matrix with musly client

This PR adds an option `-s` to the musly command line client so it can compute a sparse similarity matrix that gives the top k hits for each item (equivalent to computing playlists for all files in a collection, but a lot faster than calling musly with `-p` repeatedly because for `-s` the jukebox only needs to be initialized once). The matrix is written in the [MIREX format for sparse similarity matrices](http://www.music-ir.org/mirex/wiki/2014:Audio_Music_Similarity_and_Retrieval#Sparse_Distance_Matrix).