Proper support for source code blocks.

Only language is supported right now

Pending future work is to support other options for the jekyll code
templates.

http://octopress.org/docs/plugins/codeblock/

{% codeblock [lang:language] [title] [url] [link text] [start:#]
[mark:#,#-#] [linenos:false] %}
code snippet
{% endcodeblock %}