Unwrap divs if they only have the `nonincremental` or `incremental` classes.

```
% pandoc --incremental --from=markdown --to=revealjs
## First slide

::: nonincremental

1.  Note 1
2.  Note 2
3.  Note 3

:::

## Second Slide

1.  Note 1
2.  Note 2
3.  Note 3
^D
<section id="first-slide" class="slide level2">
<h2>First slide</h2>
<ol type="1">
<li>Note 1</li>
<li>Note 2</li>
<li>Note 3</li>
</ol>
</section>
<section id="second-slide" class="slide level2">
<h2>Second Slide</h2>
<ol type="1">
<li class="fragment">Note 1</li>
<li class="fragment">Note 2</li>
<li class="fragment">Note 3</li>
</ol>
</section>
```
