ENH: option to plot data points on top of boxplot
- [x ] tests added / passed 
- [x ] passes `black pandas`
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry - not sure how to do this. I guess I have to wait until pull request is complete.

This change is a a feature I often want, which is the ability to add raw data on top of a boxplot to get a better idea of data distribution.
