Metadata parsing for draft status and other flags #273

Pelican is designed to allow metadata in a YAML header block in source Markdown files. For example, status: draft allows you to commit drafts to the repository without them going up onto the live site.

We temporarily disabled this feature when we switched to the Marko Markdown parser. So, we need to turn it on again by writing a parser element for Marko and including it as a parser mixin in the Janeway plugin.