Unit test improvements

[JWCook]

Closes #6.

• Test both RSS and Atom output formats
• Parse XML output and compare one element at a time. This gives better test output on failure, and ignores parts that don't matter.
• Report test status in CI. Failing tests will prevent merging a PR or publishing a package.
• Add coverage report config with pytest-cov. By default, it will show condensed terminal output (total coverage). Adding --cov-report=html will generate a detailed HTML report (line coverage).
• Add some missing test coverage; now at 95%
• Run Sphinx builds in tests using SphinxTestApp.
  • This is a pytest plugin that comes with Sphinx, and is poorly documented but quite useful for testing. It does some extra environment setup that allows invoking Sphinx from python instead of subprocess(['sphinx-build', ...]).
  • You can see the full build output with pytest -vs.

Let me know if you have any suggestions, or questions about how it works!