custom palette that allowed mapping legend values to colors

[brharrington]

The current custom palettes are just a list of colors. For some use-cases it would be convenient to be able to map some labels to a given color.

Sample use-case: map http status codes to colors using green for success and reds for errors. Would likely need a default palette to use as a fallback.

The other issue with the current custom palettes is that they cannot be used with :palette operator as is.

/cc @elandau