HTML export: Export src block language as css class

Org mode exports source blocks with the language as a css-class of the exported
source block - it's easy enough to do the same.

org/testdata/captions.html

@@ -2,11 +2,13 @@
 Anything can be captioned.
 </p>
 <figure>
+<pre class="src src-sh">
 <div class="highlight">
 <pre>
 echo &#34;i have a caption!&#34;
 </pre>
 </div>
+</pre>
 <figcaption>
 captioned soure block
 </figcaption>