Enhanced Testing

[sroberts]

Trying to up my testing game to make this whole thing cleaner and easier to use.

Adding the following:

• [x] Markdown testing with Markdown Linter (fixes #212)
• [ ] Jekyll Frontmatter Testing with 18F/jekyll_frontmatter_tests (This was giving me trouble)
• [x] Reevaluate HTMLProofer Options
• [x] Check DNS with bundle exec github-pages health-check
• [x] Remove site in clean up with rm -Rf _site
• [x] Make sure current site passes all tests before merging

Much stolen from @benbalter.

[sroberts]

Couldn't get the 18F/jekyll_frontmatter_tests working. Have to find another solution.