add constant q transform

[vandenoever]

Constant Q Transform can show the volume of musical notes. mpv implements this filter.

mpv --lavfi-complex="[aid1]asplit[ao][a]; [a]showcqt[vo]"

https://ffmpeg.org/ffmpeg-filters.html#showcqt

This would be a nice visualization to run from the command-line.

[dpayne]

I think this is a good idea, if anyone wants to add it I would happily merge it. It would be added to one of the transformers here https://github.com/dpayne/cli-visualizer/tree/master/src/Transformer