Allow changing of the `checked` class name

Something like:

```js
new Checkbox(myCheckbox, {
  checkedClass: 'active',
  isChecked: true
});
```
Which would return:

```html
<span id="mycheckbox" class="active" tabindex="0" role="checkbox" aria-labelledby="jsze4iuu8c" aria-checked="true"></span>
```