Handle Metadata defined at the top of a md file

[travs]

Right now the metadata is displayed literally, between two horizontal lines.
We could handle this with showdown, and either ignore it as output, or add metadata to the html.

var conv = new showdown.Converter({metadata: true});
var html = conv.makeHtml(someMd);
var metadata = conv.getMetadata(); // returns an object with the document metadata

[adriaandotcom]

I use https://github.com/dworthen/js-yaml-front-matter for that, works only with strings though.