This makes gmnhg parse TOML/YAML/JSON/org-mode front matter in Hugo posts.
This also makes the library no longer render metadata in posts, removing
the API to do so. The metadata parsing code itself moves to
internal/gmnhg.
As the library no longer has a preset to include metadata in rendered
document, md2gmn will from now on silently discard front matter if it
encounters any.
Fixes#28. Unblocks #13.
