artists colormaps: new normalizations

[ddkohler]

Matplotlib now has good support for colormap normalization strategies that we have implemented internally (e.g. through dynamic range).

https://matplotlib.org/stable/tutorials/colors/colormapnorms.html

One minor benefit is that this would make behind-the-scenes code simpler (e.g. artists.base._parse_plot_args), but it should also make it easy for other normalizations that we have been wanting (e.g. TwoSlopeNorm).

[ddkohler]

This issue is related to #38