[Feature] Support markdown with metadata

[grgprarup]

Pull Request Description

This PR makes changes to the markdown plugin to support metadata in markdown. Also, adds example templates, CSS, and unit tests.

Added:

• Supports markdown files with metadata
• Two types of metadata
  • Default (in frontmatter format)
  • Inline (in yaml or yml code block), specific to the slide
• Templates example (<project_base>/templates)
  • templates/cover-template.html
  • templates/title-content-template.html
  • templates/titile-image-template.html
• Markdown example for test (<project_base>/test)
  • test/metadata-markdown.md
• CSS example (<project_base>/css/theme/source)
  • css/theme/source/metadata.css
• Used libraries/packages
  • front-matter - to parse default metadata
  • js-yaml - to load inline metadata
  • mustache - template system

Modified:

• Markdown Plugin (<project_base>/plugin/markdown/plugin.js)
  • Functions added
    • parseFrontMatter - to separate default metadata and the rest of the content
    • separateInlineMetadataAndMarkdown - to separate inline metadata and the rest of the content of each slide
    • renderTemplate - to render content with a template
• Unit test added (<project_base>/test/test-markdown.html)

Related Issue:

• Part of [Feature Request] Support Markdown with metadata #3462

Screenshots:

• Cover slide [cover-template]
  
• Title Content slide [title-content-template]
  
• Title Image slide [title-image-template]
  

Checklist:

• Code follows the project's coding style and best practices
• Code has been reviewed by team members
• Documentation has been updated to reflect the changes

Discussion

I am sorry to mention you but I need your thoughts on this.
CC @hakimel

[quinnypig]

@hakimel I just spent a fair bit of time looking for exactly this functionality; is there a reason not to merge this that'll blow up in my face if I use it? Alternately, is there a better answer that might be lurking around somewhere in the codebase? "Having a series of slide templates that I can grab and populate with content" is kinda the dream here.

[individual-it]

@quinnypig we have pulled the functionality into an own plugin. Have a look at https://github.com/JankariTech/revealjs-awesoMD

[dude-at-RA]

@quinnypig have you looked into using Pandoc with reveal? With YAML frontmatter in your markdown, you can do a LOT of templating. You can even bring your own RevealJS targeting CSS pretty easily too.