Metadata at start of Markdown should be in standard YAML Front Matter (at least optionally)

[vorburger]

The id: etc. "Metadata" documented here currently (quote) "must come before the title".

This is unusual by Markdown "standards" (as far as there is such a thing; I know!); typically such "headers" are put into "YAML Front Matter". Some other Markdown parsers can ignore it as text, but extract it for processing. GitHub (now) renders this "nicely", see https://github.blog/2013-09-27-viewing-yaml-metadata-in-your-documents/.

So instead of e.g. this:

id: my-codelab

# My Codelab

claat should, at least optionally (for backwards compability) support this:

---
id: my-codelab
---

# My Codelab

It currently doesn't, and fails, saying: codelab.md invalid metadata format, missing at least id: map[duration:0:02:00]