Sorting out how to add external libraries in the HTML header

[rsoden]

Right now this is hardcoded in the layout.jade file in vizdown. Each js plugin file (for maps or whatever else) should have the ability to include external libraries or css assets in the overall header of the page. Leaflet, for example, requires:

link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.css"
script src="http://cdn.leafletjs.com/leaflet-0.7.3/leaflet.js" /script