ENH Series.getattr for attributes lacking built-in accessors
Adds a `Series.getattr()` method for accessing attributes from a Series of objects that do not have built-in accessors like those in `Series.str` or `Series.dt`. This is faster than mapping/applying a `lambda obj: obj.attr` function because it uses `operator.attrgetter` and is also syntactically concise, similar to other accessors like `series.imag` or `series.dt.date`.

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