ENH: `Styler.text_gradient`: easy extension alternative to `.background_gradient`
This is minor code alteration to the `_background_gradient` function that permits a text-only coloring version, i.e. `Styler.text_gradient`.

This could alternatively be performed with a `text_only=True` keyword addition to `Styler.background_gradient` existing method, but it seemed dissimilar enough to merit its own method (and avoid adding more kwargs).

![Screen Shot 2021-04-22 at 14 24 45](https://user-images.githubusercontent.com/24256554/115713844-bef30c80-a376-11eb-94cf-62b4c63aa4de.png)
![Screen Shot 2021-04-22 at 14 24 57](https://user-images.githubusercontent.com/24256554/115713854-c1edfd00-a376-11eb-8819-ab1b6b918564.png)


lgtm. can you rebase one more time and ping on green. also pls update  the styler docs as a followon. @jreback pinging green now. Thanks @attack68 