Add author and keywords directives and CLI options for setting metadata

[yhatt]

Added author and keywords global directives and CLI options for setting metadata in the conversion output.

---
author: Yuki Hattori
keywords: marp,marp-slide,marp-cli
---

• author: Set the name who made the slide deck. It is available in HTML, PDF, and PPTX.
• keywords: Set comma-separated keywords for the slide deck. It is available in HTML and PDF.

  • In HTML, it can set <meta name="keywords"> tag. However, it is known as no longer used by Google since 2009. Don't expect improvement of SEO by setting keywords.

  • In PDF, keywords will store to PDF metadata by turning into semicolon-separated keywords, that is the best practice of PDF keywords. e.g. marp,slide,marp-cli -> marp; slide; marp-cli

  • keywords also accepts an array of strings in the global directive and Marp CLI configuration file.

    ---
    title: Marp slide deck
    author: Yuki Hattori
    keywords:
      - marp
      - marp-slide
      - marp-cli
    ---

    or

    marp slide.md -c ./config.json

    // config.json
    {
      "title": "Marp slide deck",
      "author": "Yuki Hattori",
      "keywords": [
        "marp",
        "marp-slide",
        "marp-cli"
      ]
    }

Resolves #367.

ToDo

• Add tests
• Update README.md