added ability to add formatted markdown to a section of migradoc document

Hi-

this allows the addition of Markdown to an existing migradoc doc.

The driver for this is a log analysis tool that I'm working on; internally it creates 'analysis notes' that are formated in Markdown; adding that with this change means I can generate a pdf document and iterate over all the notes analysis notes and add them to one pdf document.

I've just realised that this also will allow the inclusion of markdown into any document type that migradoc can export to.