discrete option for continuous palettes

[matthewstern]

From @gritzenthaler:
Maps are beyond the scope of this project, but perhaps someone might have a graphical need for displaying a continuous palette in binned form? Do you think it's worth taking the time to add a "discrete" argument to the functions? (The Viridis source code is here for reference: https://github.com/sjmgarnier/viridis/blob/master/R/scales.R. I'm poking around in it now.)