ENH: Explode multiple columns of DataFrame
Now .explode() can take a list of column names and will explode multiple at the same time (given that each element across all the columns have the same length in every single row
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @stahl085! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-09-16 16:48:53 UTC Just a heads up - this was part of the original implementation in #27267 so you can check there for inspiration on tests and implementation. The big blocker there though was how to handle duplicate values, i.e. whether we should generate a cartesian product or not. Do you know how other similar tools would handle that? Thanks for the info, I like that #27267 implementation much better!
By duplicate do you mean the user passes in the same column name twice? ex: ['A', 'A']

I haven't seen this implemented elsewhere, but to me it seems un-natural for this to return a cartesian product. Would it make sense to include that as an optional argument `cartesian=False`?