Add support of heylogs configuration in Markdown front matter

[charphi]

Allow heylogs configuration to be embedded directly in the changelog file's YAML front matter, as an alternative to heylogs.properties.

A CHANGELOG.md with front matter:

---
heylogs:
  versioning: semver
  forge: github
  tagging: prefix:v
---

# Changelog
...

would be equivalent to a heylogs.properties in the same directory:

versioning=semver
forge=github
tagging=prefix:v