Tests involving Pygments are fragile

[jonafato]

Certain tests involving Pygments (and possibly other dependencies) are fragile and can cause test to fail with minor version updates (see #314 for an example). Something should be done to wrap the content generated used to test Hyde's output to make this less error prone. Ideally, this would be done for all tests that currently hard code output generated by dependencies (e.g. pygments, docutils, markdown, etc.).