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/blocks.html

@@ -1,4 +1,5 @@
 <figure>
+<pre class="src src-bash">
 <div class="highlight">
 <pre>
 echo &#34;a bash source block&#34;
@@ -10,15 +11,18 @@ function hello {
 hello
 </pre>
 </div>
+</pre>
 <figcaption>
 block caption
 </figcaption>
 </figure>
+<pre class="src src-text">
 <div class="highlight">
 <pre>
 a source block without a language
 </pre>
 </div>
+</pre>
 <pre class="example">
 an example block with
 multiple lines including
@@ -120,11 +124,13 @@ this unindented line is outside of the list item
 <p>
 list item 2
 </p>
+<pre class="src src-text">
 <div class="highlight">
 <pre>
 #+BEGIN_EXAMPLE
 </pre>
 </div>
+</pre>
 <p>
 #+END_EXAMPLE
 </p>