Merge pull request #7 from dominikschnitzer/guess_neighbors_filtered

Filtered version of musly_jukebox_guessneighbors()
Fixed memory reallocation on each decoded frame
Support planar samples in libav decoder
Fix memory leak in libav decoder
Allow specifying the length and position of the excerpt to use in musly_track_analyze_audiofile(); strongly improve the performance of the libav-based decoder by decoding only what's needed
Regenerated documentation to reflect API change
Update README to reflect API change