ENH: add a gradient map to background gradient
closes #22727
closes #28901
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

A very minor extension to the code and adding one argument gives the function a lot more scope:

![Screen Shot 2021-02-20 at 14 04 14](https://user-images.githubusercontent.com/24256554/108596326-a0779180-7384-11eb-97f5-60ea0a23bda1.png)

