Add support to the existing marked markdown preset for footnotes

it turned out to be trivial to add GFM footnote support with marked-footnote to the existing marked markdown preset. I suppose this could be optional, but I'm not sure how to add in an option to the preset plugin's config, though if such an option existed, it would be easy to change the way marked is called based on that option. As it stands, with this patch, it is on all the time.