downlit does not keep code source option set with knitr

[cderv]

knitr supports class.source and attr.source to add class and attributes on fenced code blocks

Take this example

---
output: 
  rmarkdown::html_document:
    highlight: pygments
---

```{r select1, class.source = "my-class"}
print("Hello there")
```

This is the markdown produced by knitr

```{.r .my-class}
print("Hello there")
```

and my-class will be added by Pandoc to <pre> in code block

<div class="sourceCode" id="cb1">
<pre class="sourceCode r my-class"><code class="sourceCode r">
<span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">print</span>(<span class="st">&quot;Hello there&quot;</span>)</span>
</code></pre></div>

downlit is not saving the addition classes to put them back in the new <pre> that it creates.

Related to #133 which is slightly related as order of classes on pre messes up downlit_html_path().

A fix should take into account both issues probably

Initially reported in r-lib/pkgdown#2147 by r-lib/pkgdown#2147