automated testing of html output

[jseldess]

We should put in place an automated test on Jekyll output. Ideally, the test would run immediately following jekyll serve or jekyll build. At a minimum, the test should catch broken links and broken images.

This might do the trick: https://github.com/gjtorikian/html-proofer

[jseldess]

Installed and ran against local html output. Found a few broken links. Also added some alt-texts. For links in topnav, which the proofer doesn't like the format for, added the data-proofer-ignore attribute.

I think this will be the command to use going forward (from root directory of branch):

$ htmlproofer ./_site --allow-hash-href true

--allow-hash-href true excludes all on-page anchor links from the proofer.