ENH: Styler.bar height control
- [x] closes #36419 
- [x] tests added / passed
- [x] whatsnew entry

I originally thought the issue was too complicated to code for such a small feature, but turns out it was only one or two lines of coding, so I added it in. Quite a nice addition for visualisations I think.

![Screen Shot 2021-07-11 at 17 23 59](https://user-images.githubusercontent.com/24256554/125200903-309f7000-e26d-11eb-8ae8-1b12d83523e4.png)
