Update documentation for colormaps

[kimikage]

This adds visual examples for sequential_palette.

Closes #265

[kimikage]

I had originally planned to update the documentation after #509. However, now I think it's better to update the documentation first to check the effect of the hue interpolation fix in #509.

[kimikage]

Probably, ColorSchemes.jl is the most comprehensive and widely used package in visualization. As a matter of fact, we no longer have situations where we must use the colormap. Also, modern perceptual color maps, like "Viridis", no longer use the Luv color space. (Even the CAM02-UCS is already becoming outdated.)

So, I think we should regard colormap as a "simple" tool which can be used without loading the plot library.
I'm also starting to wonder if it might be a good idea to add predefined diverging colormaps, mimicking ColorBrewer (i.e. "Spectral", "RdYlGn", "PRGn", "PiYG", and "BrBG").