Add markdown support for head metadata

[Eakam1007]

This could include:

• title
• stylesheets
• scripts

Markdown could look like this and would only work if placed at the beginning of the markdown file:

---
title: Document title
stylesheet: stylesheet_link1
stylesheet: stylesheet_link2
---

This markdown sets the title, and adds links to two stylesheets.
(Perhaps something better can be used for multiple stylesheets)