Translations of values for fill and color

[ijlyttle]

This concerns when we specify a value, as opposed to a field (variable) to map colors:

• ggplot2 accepts R colors: colors()
• Vega-Lite accepts a css color

Fortunately hex colors, e.g. "#663399" are valid in both systems. The named colors also seem to be very similar - they both seem to derive from X11 colors.

Proposal for the TypeScript side of things:

• if the value begins with a # and has six characters following, let it through as-is.
• if the value does not begin with a #, remove any trailing numbers from the color.