ENH: add `decimal` and `thousands` args to `Styler.format()`
A PR for Europeans:

![Screen Shot 2021-03-23 at 21 40 19](https://user-images.githubusercontent.com/24256554/112216306-a1b90a00-8c21-11eb-9d2b-c281154b4b46.png)


thanks @attack68 
can you open an issue (and a PR if you can!) to try to unify the string formatting for these  (and strings) at some point. thanks!