Add support for info headers

[greduan]

@nelstrom Lately static site generators have gotten into using a header inside a Markdown file to give the static site generator the file's metadata.

So something like this:

---
title: Contact
layout: page
headline: Contact
subHeadline: Drop me a line or two
menuShow: true
isPage: true
menuOrder: 2

---

Of course this causes problems with this plugin, where it thinks the last line is a header.

You think you could fix this because it's a little bit annoying. :)

[nelstrom]

This was reported by @matthias-guenther in #issue 3. Please continue the discussion over there, I'm closing this as a duplicate issue.