Fenced Block Types, Generic Extension/Webcomponents, and fallback handling

Rather than using code blocks for rendering (which are intended for source code), I suggest we use the concept of “hybrid” documents containing special sections that the parser skips over but may be recognised by other applications. That way we’re not introducing yet another syntax and concept to learn. Refer to @jgm’s comment here:

http://talk.commonmark.org/t/metadata-in-documents/721/53