Create intermediate DOM from parsed markdown

[tobiastom]

It have more influence on the generated output, it would be nice to have an intermediate DOM like object structure, that could easily be changed. This would for example allow an PDF output format, that uses this DOM tree and create a file as it needs it. Another format use case might be RDF.

Another use case would be to change elements inside this tree based on the context. For example users could write plugins that iterate on the tree to add line number to code blocks.