ENH: add NDFrame.select_str
- [x] closes #26642
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Adds a method ``select_str`` to supplement ``select_dtypes``. This PR is intended to be followed by a PR that deprecate ``NDFrame.filter``.

I miss the most having a string focused axis-filtering method, and is ``NDFrame.filter`` a bit unfocused  and badly named IMO. I think usage of parameter ``items`` in ``NDFrame.filter`` can just be replaced by recommending ``NDFrame.__getitem__`` and parameter ``like`` is less useful than ``startswith`` and ``endswith`` (is less precise). So this is not a 100 % replacement for ``NDFrame.filter``, but intends to make something that is more useful (IMO, of course).

This is not fully tested and need a whatsnew etc. but I would like to get some response before I go further.

Hello @topper-123! 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 2019-07-11 20:38:22 UTC Closing in favor of #27363