Customizable squashing function

Mapping [0, ∞) to [0, 1] can be done many different ways, and I found [some worked better or worse for different functions](https://flic.kr/p/CQibRd).

I originally used `mpmath.cplot` for this, which combines squashing and colormapping into the same function `default_color_function()`, but maybe more logical to separate them? Like specify mapping and colormap as parameters to `cplot`, like matplotlib's `norm` parameter.