Add coloured waveforms

It would be cool to implement the coloured waveform (freesound-style):

![image](https://user-images.githubusercontent.com/114316/101288703-3f660600-37f8-11eb-860b-13450b2278e8.png)

The code is available in the [Freesound](https://github.com/MTG/freesound/tree/master/utils/audioprocessing) repository (in the wav2png.py script, I guess). 