Fenced setext headings

[chrisalley]

There is a discussion about adding Flowerbox headings to CommonMark in this topic:

Flowerbox Headers on top and bottom of a header

This is a minor annoyance, but I would like ============== Header Title ============== -------------- Header Title -------------- to be recognized as: Header Title Header Title not: ============== Header Title Header Title I use this in certain context, where it stylistically makes sense. For example you would use this in a resume. It makes a long document easy to scan with the eye, since you can recognize the double header as being a major section break.

This seems like a good feature to add to a Markdown successor language. I’m not sure how much value there is making this a CommonMark extension given that it will break existing documents and mainly exists for aesthetics.