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 needed a markdown parser that would return a structure I could iterate on. Since your project is well established I thought it was a good idea to use it. I browsed all the documentation about using and extending your module, but since I don't intend to output HTML, I can't write an extension plug-in.

I may have missed something, but I failed at finding an entry point that would give me just what I need. Ultimately, I did it myself. I've split the convert method in two steps, parse and serialize, so I could use only the parse method and call it a day. I submit you this pull request in case you would be interested in my modification (and, that would be great news for my project to avoid packing the lib with it). Developpers could re-use this entry point to plug Markdown language to other forms of output (like, for example, wiki markup).

Looking forward any return on this.