Motivation
When highlighting GitHub's issue or PR templates or markdown files for Jekyll, the meta info about those files isn't highlighted.

Description
Front matter is a short section of meta-information in YAML (or similar languages) about a file and usually looks like this:

---
layout: post
title: Blogging Like a Hacker
---

For now, it should be enough to support YAML front matter as that seems to be the most common.