WIP - HTML validation

Putting this out there to see what people think.

This uses [tidy-html5](https://github.com/safarijv/tidy-html5) to validate __offline__ the markup of a small selection of pages.

__NOTE:__ This command must be built locally for the tests to run. We could add a conditional tag to these tests so that they're only run in environments containing the binary. A stretch goal would be to package it up as a gem with native extensions :neckbeard: 

So far the home page, a category and an article page are validated, using both locales.

@andrewgarner @pvcarrera @jongilbraith