Split convert method in two steps

Hi Waylan

I am currently writing a project that will use markdown input files (more precisely, a spritz-like reader, that would be aware of text structure like paragraphs, titles etc.), and I need a markdown parser that would return me structure I can iterate on. I tried to use your implementation to do so. I browsed the documentation about using and extending your module but I failed at finding an entry point that would give me what I need. Most of the documentation cover writing extension plug-ins, which I can't use since I don't intend to output HTML.

Ultimately, I hacked an entry point myself. I've split Markdown.convert method in two steps, parse and serialize, so I can extract the tree from the parse method. I also aliased convert to be serialize(parse(text)) to keep API compatibility.

I submit you this pull request in case you would be interested in my modification. Developers could re-use this entry point to plug Markdown language to other forms of output (like, for example, wiki markup). Please let me know if there was any cleaner workaround, or if you consider this change to be worth including in your project.