Add visual comparison tests

[Dan Foreman-Mackey]

We don't currently test how the output PDFs actually get rendered. Instead, we only test that the build runs successfully. It would be great to add some tests to make sure that our changes don't break the expected formatting. Perhaps we could use something like matplotlib's compare_images test function to get this to work: https://github.com/matplotlib/matplotlib/blob/fbe7a446a60e603e44a7a6fbdd77f2be39d6796c/lib/matplotlib/testing/compare.py#L384