Notes plugin: data-note does not work on slide-level

We have 2 different ways to specify notes: with `data-notes` attribute and also with separate tag `<aside class="notes"`. But former does not work for a slide(but works at fragment level btw)

Example:

```html
<section>
  notes will be shown
  <aside class="notes">slide-level notes</aside>
</section>
<section data-notes="slide-level notes">
  notes will NOT be shown
</section>
```

