Add code filepath to code blocks

This is something I've needed forever when writing books and training
material with code samples: the ability to easily list where a file
should go on the reader's machine:

e.g.

input
~~~
``app/models/person.rb
class Person
end
``
~~~

output
```
<div class="code-filename">app/models/person.rb</div>
<pre><code>
  ...
</pre></code>
```

Barring putting it into the parser directly, being able to override the
kramed callbacks before render would be nice (probably better).