Add support for non-monotonic colormaps #20
Description
There are important classes of colormaps, including divergent and rainbow-like maps that are not monotonic in lightness. Supporting these would make the site more useful.
It would be:
- One curve (set of control points)
- Multiple segments: each segment is monotonic in lightness, and the direction of monotonicity alternates for each segment
- Breakpoints have an associated value, so that value -> lightness becomes value -> segment -> lightness
- Button to break a colormap: reverses lightness monotonicity