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/lists.html

@@ -59,11 +59,13 @@ and some lines of text
 <p>
 and another subitem
 </p>
+<pre class="src src-sh">
 <div class="highlight">
 <pre>
 echo with a block
 </pre>
 </div>
+</pre>
 </li>
 <li>
 <p>
@@ -131,11 +133,13 @@ term<dd>
 details on a new line (with an empty line in between)
 <strong>continued</strong>
 </p>
+<pre class="src src-bash">
 <div class="highlight">
 <pre>
 echo &#34;Hello World!&#34;
 </pre>
 </div>
+</pre>
 <dd>
 </dl>
 <p>