Fix/cdodge/incorrect html serialization

@cpennington @ormsbee @cahrens can one of you review?

We got a bug report where someone tried to export an HTML module like the one in with_styling.html (test file). Basically the HTML definition_to_xml() first tries to just treat the HTML as an XML block, but that causes all of the stuff in the 'p' HTML tag to get dropped in the export.

My fix is to always write HTML content into a separate .html file