Add column index to be removed in get_dummies
While create dummies with get_dummies, we sometime want to remove an other column than the first one.
Add this possibility by :
- changing drop_first to drop_idx,
- drop_idx accept both bool (as drop_first) and int

 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry
