Permit putting class members on 'stub' pages

[lukelbd]

This PR addresses item 3 from #91. It permits using "stub pages" for classes attributes and class methods, controlled by the automodsumm_stub_pages option. The default is False.

It inserts process_automodapi via app.connect with a higher priority than autosummary, so that autosummary can generate the stub pages referenced by the TOC tables in the automodsumm-generated class pages, and adds the automodsumm-generated pages to the env.found_docs list where autosummary can read them.