API: PeriodIndex subtraction to return object Index of DateOffsets
Implements _sub_period_array in DatetimeIndexOpsMixin.  Behavior is analogous to subtraction of Period scalar.

- [x] closes #13077
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
