Add the default monospace font stack for <code> elements

Unfortunately <code> elements weren't getting the monospace font stack they
deserved so they looked a little out of place. :lipstick:
Updating to use GitHub default font stack

- Make the <code> font stack more consistent from GitHub.com.
- Refactor code/pre selector rules to be generic enough to catch all
the lingering code blocks.
Merge pull request #427 from jacobbednarz/update-code-blocks-to-use-correct-monospace-font

Add the default monospace font stack for <code> elements