[opinionated suggestion] Default colorscheme same as LibROSA

[ftrebien]

I'm using an evenly-spaced subset of the colors from matplotlib's perceptually uniform Magma colormap, used by LibROSA by default and similar the spectrograms of some other famous pro audio software. It looks really good, so I'd like to suggest that as a default. Even better would be to use all the 256 levels. Another option would be the Viridis colormap (same links), which is similar to the current default color scheme.