Adding accessibility support

Adds aria role of ```progressbar``` and exposes an ```alt``` attribute that will be used as ```aria-label```.
```alt``` defaults to ```loading``` if not specified, not great for internationalization but in my opinion better than the alternative which would make it hidden from screen readers.
Also setting ```aria-hidden``` when active state changes.

Don't set `aria-label` if it already had that attribute and just set `this.alt` to its value:

```javascript
if (this.hasAttribute('aria-label')) {
  this.alt = this.getAttribute('aria-label');
} else {
  this.setAttribute('aria-label', this.alt);
}
if (!this.active) {
  this.setAttribute('aria-hidden', 'true');
}