Add a case: leading paragraph in fenced_code_blocks.

[keysona]

Hi, I met a problem when I want to convert my article written by others markdown editor like Dillinger.

The problem in code block.

The input:

This is python code.
```python
import os
print(os.listdir('.'))
```

The output:

<p>This is python code.
<code>python
import os
print(os.listdir('.'))
</code></p>

But expected output is this:

<p>This is python code.</p>

<pre><code>import os
print(os.listdir('.'))
</code></pre>

I think most markdown editor suppor this, but markdown2 don't.
So I try to support this.
I have add some test, and all others test has been pass.

[nicholasserra]

Thank you :D