Support the [source%nowrap] attribute on a source block

[xcoulon]

Eg:

[source%nowrap,java]
----
public class Foo {
}
---

should include a nowrap class in the <pre> element:

<div class="content">
<pre class="highlight nowrap"><code class="language-java" data-lang="java">public class Foo {
}</code></pre>
</div>
</div>