Add u-hidden visibility util and update comment style.

We’ve had multiple requests for an equivalent to the old `x-hide` class; this adds a new equivalent.

Status: **Ready to merge**

Reviewers: **@kpeatt, @jeffkamo **
Ticket: **Fixes #38 **

## Changes
- Applies the util-level (high) specificity to the `hidden` attribute. The hidden attribute is preferred to a `u-hidden` class because it exactly matches our intent and has machine-readable semantics. No sense re-inventing the wheel.
- Does *not* add a `u-shown` class; I haven’t heard of a need for this, and there doesn’t seem to be a use case.

## Research resources
- See [HTML5 boilerplate](https://github.com/h5bp/html5-boilerplate/blob/master/dist/css/main.css)’s css for their take on this.