Add `[hidden]` selector counteract all `display` declarations

Anytime a `display` declaration is used in Pure with a value other than `none`, we need to add an additional rule with similar selectors (appended with `[hidden]`) with a `display: none;` declaration. This allows the `hidden` HTML attribute to work properly on elements styled by Pure.

The following button will be hidden from view with these changes:

```html
 <button class="pure-button" hidden>No showy</button>
```

This is an initial pass at applying this practice to Pure's source. We should discuss this and make the selectors used smarter and probably clean them up. Overall this **added 0.1KB** to Pure's size.

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Fixes #177