ENH: Allow where/mask/Indexers to accept callable
- [x] closes #12533
- [x] closes #11485 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Current impl is very simple like `assign`. Another idea is applying some restriction to return value from `callable` (for example, only allow aligned `DataFrame` or scalar).
