The generation of sphinx config has been consolidated into a single entry point: `jupyter_book.config:get_final_config`, where before it was spread over yaml.py and sphinx.py, making it difficult to determine what the finaly configuration would be.

This now allows for proper testing of the configuration generation, added in tests/test_config.py, which also clarified some bugs identified in #915. These have now been fixed.

Finally this has also allowed for the addition of a `jb config sphinx` command, which generates the a Sphinx conf.py representation of the build configuration.
This should help advanced users/maintainers to understand/debug builds.