Run doctests as part of `python setup.py test`

In an online discussion with @keflavich, we discovered that doctests were not running as part of `python setup.py test` in astroquery.

This resolves that by

1) turning on the doctest_plus plugin in `setup.cfg`

2) delegating from the `pytest_configure` function in astroquery to the one in astropy, which sets up all the doctest magic.

@embray: Is the `setup.cfg` requirement documented somewhere?  Should it be, and if so where?

@astrofrog: Does this have any impact on the package template?