ENH: `Styler.bar` accepts matplotlib colormap
No one has requested this but I think its nice none-the-less, especially in finance / science. Kind of like background_gradient with slightly different vizualisations.

![Screen Shot 2021-09-19 at 21 25 07](https://user-images.githubusercontent.com/24256554/133940391-b2a8157e-8346-4cb4-8af2-8e71146e7c90.png)


![Screen Shot 2021-09-19 at 21 32 01](https://user-images.githubusercontent.com/24256554/133940536-8cc11271-0564-4123-a79e-b715d61356d8.png)


