Gemini spec p. 5.5.1 used to only allow up to three #-s in a heading
before requiring a mandatory space. It changed to an optional space in
recent updates, allowing to no longer pad Markdown H4-H6.
As clients treat everything after ### a title continuation, the renderer
will now insert H4-H6 verbatim; the end-user behavior doesn't change as
extra space means nothing for a Gemtext renderer displaying the title in
a special way.
Relates to #1.
PS: Gemini spec doesn't appear to be properly versioned, saying the
latest version is 0.14.3, 2020-11-29. The discussion on #1 clearly shows
it used to be different a while ago.
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.
