ENH: `Styler.format_index()` to display index values similarly to data-values with `format()`
- [x] adds features and method with docstrings
- [x] update user guide
- [x] whats new
- [x] add tests

This PR adds the method `format_index` so that the typical `format` routines can also be applied to display the index labels and columns headers of a Styler.

With all the recent functionality here is a complete example:


![Screen Shot 2021-08-18 at 22 55 02](https://user-images.githubusercontent.com/24256554/129970916-0092bd76-f322-4b5f-87b4-1276ac2d303e.png)



**Notes**

Also had to move `_refactor_levels` function from `style.py` to `style_render.py` for use here.
