Include special methods with automodapi?

[jonathansick]

We want to include special methods (i.e., dunder methods) in the API references that we make through [automodapi](https://github.com/astropy/sphinx-automodapi/]. Sphinx autodoc supports with with a special-members option:

.. autoclass:: my.Class
   :members:
   :special-members:

It looks like the place to implement this is https://github.com/astropy/sphinx-automodapi/blob/master/sphinx_automodapi/automodapi.py#L308-L312 I think we can PR to sphinx-automodapi to add a private-members option to automodapi that's passed through to the autoclass directives.

[jonathansick]

Possibly this could be implemented simply through the autodoc_default_flags configuration.