Nice to be able to get a list of colors from a color scale object

[alshan]

For example: scale_color_hue(c=20, l=90).palette(20) would generate a list of 20 color codes.

This list can then be used in scale_color_manual() to achieve synchronous coloring across multiple plots.

[alshan]

Related: #780

[alshan]

See demo: https://raw.githack.com/JetBrains/lets-plot/master/docs/f-26a/scale_color_palette.html