Support build_sphinx setuptools plugin

[jaraco]

It would be nice if RTD could support the setuptools build_sphinx plugin for building docs for projects. For example, the setuptools project itself uses the build_sphinx plugin to build its docs, which allows the project to also define parameters for doc building in the setup.cfg file.

[ericholscher]

We try to avoid doing anything language specific, and this seems like a Python-specific feature.