.md header parsing is very strict

[daniel-v]

Should a page.md file contain a header, the format is expected to be in the exact form of:

---\nCONTENT\n---\n

We could:

• document that this exact format is expected
• loosen the requirements so that:
  • header boundary token matches r-{3,}, so that ------- is just as valid as ---
  • optional empty newlines before opening --- is consumed: [\r\n]*-{3,}
  • having the same number of dashes in opening and closing is not a requirement

I consider this a low-prio issue.