Support markdown front matter

[RunDevelopment]

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.

[mantou132]

import fm from 'front-matter';

const { frontmatter, body } = fm(md);

const resultCode = Prism.highlight(frontmatter, Prism.languages['yaml'], 'yaml') + Prism.highlight(body, Prism.languages['md'], 'md')

[mantou132]

When will it be published to npm?

[RunDevelopment]

Next release will be in January.