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

Anytime a Pure style rule uses the `display` declaration with a value other than `none`, using the `hidden` HTML attribute on that element won't have the desired affect of hiding that element. To counteract this, A new rule has been added to Base to force `display: none !important;` on elements that use the `hidden` HTML attribute.

The following button will be hidden from view with these changes:

```html
<button class="pure-button" hidden>No showy</button>
```

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Fixes #177