make codeblock play nice with HAML

The `{% codeblock %}` tag trigger a HAML parsing error when both the lang and a description is given. Here is a sample post example:

```
---
layout: post
title: "test"
date: 2013-05-11 15:49
comments: true
categories: 
---

{% codeblock there is no escape lang:c %}
int
main(void)
{
  return (42);
}
{% endcodeblock %}
```

Running `rake generate` will generate the following error:
```
!!! HAML Error: Illegal nesting: nesting within plain text is illegal.
```

The proposed patch is fixing this particular issue.