[Enchancement] Allow for usage of special color variables

Currently, there doesn't seem to be a way to use the `background`, `foreground`, and `cursor` variables, and the config is limited to colors 0 through 19 (which seems odd, since pywal only generates 15 + 3 colors)

pywalfox should allow for selection of those 3 variables, on top of  the standard 16 (or 20?) color variables, 