Add option to disable slides 

See #428 - needed to clean that up anyway, here's the PR.

```html
<div class="step" data-y="100">
    I'm step 1!
</div>
<div class="step disabled" data-y="200">
    I am fully visible, but not selected when navigating via arrow keys!
</div>
<div class="step" data-y="300">
    I'm step 2!
</div>
```

Cheers,
Max