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/footnotes.html

@@ -70,11 +70,13 @@ and other elements
 <p>
 like blocks
 </p>
+<pre class="src src-text">
 <div class="highlight">
 <pre>
 other non-plain
 </pre>
 </div>
+</pre>
 </li>
 <li>
 <p>