HTML5 output mode

```
In patterns (see http://patternslib.com ) we use pagedown to render markdown
and insert it into an existing page. In order to get header levels correct we
want to use the preferred HTML 5 approach: instead of creating h1-h6 elements
we use nested sections, each of which has a h1.

In order to do this I modified pagedown to take a new rendering mode parameter
which is used to tweak the header conversion.
```

Original issue reported on code.google.com by `wich...@outlook.com` on 1 Feb 2013 at 3:06

Attachments:
- [html5-output.diff](https://storage.googleapis.com/google-code-attachments/pagedown/issue-51/comment-0/html5-output.diff)