I really should've paid more attention before submitting #660...

This PR finishes off what #660 should've done by excluding test.html from the repository's language breakdown. It's common practice in many GitHub repos to mark test files with the linguist-documentation attribute.

This is identical to linguist-vendored in that it stops marked files from skewing language stats... the key difference being that "documentation" files are included in diffs on GitHub, whereas vendored files aren't. Test-related files are the sort of thing that you don't want bloating language stats, but don't want hidden in diffs, either.

I know this isn't a huge issue, but if I'm gonna contribute a menial fix, I may as well do it right.

Here's the output of running linguist in the updated repository:

λ Normalize.css: linguist --breakdown .
100.00% CSS

CSS:
normalize.css

λ Normalize.css (linguist-fix-part-II)

Whereas before, it was this:

λ Normalize.css: linguist --breakdown .
65.83%  HTML
34.17%  CSS

CSS:
normalize.css

HTML:
test.html