Embedding HTML5 bundles

OU-XML allows HTML5/JS/CSS apps to be embedded in an IFrame within a VLE page. The app needs to be provided in tthe form of a zip file and MUST contain an index.html file at the root.

```{ou-html5} path/to/html5.zip
:width: 140
:keep: always
```

Currently, this does not work with Sphinx rendering to HTML.

This extension also provides machinery that is used by serveral other extensions (for example, ou-mol3d and ou-codestyle).