Support setting HTML metadata

[yhatt]

This PR adds the support to recognize HTML metadata from Markdown front-matter, powered by custom directives support in Marpit v0.7.0.

---
title: Slide deck
description: A beautiful slide deck created by Marp
url: https://marp.app/example
image: https://marp.app/ogp.jpg

theme: default
---

# Hello!

Marp CLI will recognize 4 new global directives.

• title: Setting title for <title> tag and OGP title.
• description: Setting description for <meta name="description"> and OGP description.
• url: Setting canonical URL for <link rel="canonical"> and OGP URL.
• image: Setting OGP image URL. Twitter's Summary Card will be used the large image by setting image and title.

You can also use HTML comment like Marpit global directive for setting meta values, but it would not common usage.

We also added CLI options for setting these values. It is useful within building HTML in CI and Netlify, to use canonical URL getting from environment variables.

marp --url $URL slide-deck.md

// marp.config.js
module.exports = {
  url: process.env.URL,
}

ToDo

• Add tests
• Update docs