Labal Text based on Checkbox Status

I added the ability to make it so you can easily change the label text based on the status of the checkbox. It does require a slight change to the html layout.

There are two ways to accomplish this.

First way, using two additional span tags within the main one
```html
<div class="checkbox checkbox-slider--X">
	<label>
		<input type="checkbox" name="pb-page-status" id="pb-page-status">
		<span>
			<span class="checkbox-slider-checked">Public</span>
			<span class="checkbox-slider-not-checked">Private</span>
		</span>
	</label>
</div>
```
Second way. using one additional tag and data attributes.
```html
<div class="checkbox checkbox-slider--X">
	<label>
		<input type="checkbox" name="pb-page-status" id="pb-page-status">
		<span></span>
		<span class="checkbox-slider-label" data-checked="Public" data-not-checked="Private"></span>
	</label>
</div>
```

Thanks for this really cool CSS solution to toggles!