Add option to make use of GitHub's official markdown parsing

[KrauseFx]

To make sure it's fully compatible, we could make simple web requests to the GitHub markdown API

This will generate just the HTML code, not the CSS necessary

[phseiff]

In case you want to also use the GitHub CSS, you could get it from here. You will need to fill the HTML generated by GitHub's markdown API into a blueprint to get stuff like the border around the page, which isn't done by the GitHub markdown API itself.

I actually have a project which does exactly that, so you could take a look at how it does it if you want some inspiration. The src/prototype.html file and the src/github-css.css file in this repository should contain the HTML for the classical GitHub-README-border as well as the CSS required to style the HTML generated by the GitHub API.