Support language name as attribute on fenced code block start line

The following syntax should now be valid:

    ```lang
    CODE HERE
    ```

Which generates:

    <pre><code class="lang">CODE HERE
    </code></pre>

This syntax allows support for [GitHub Flavored Markdown][1] style fenced code blocks used on GitHub.

[1]: https://help.github.com/articles/github-flavored-markdown#syntax-highlighting