Add support for non-monotonic colormaps

[Mangara]

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