commonmark 0.31.0

• Relicense npm package under CC-BY-SA (Yukai Chou).
• Use archived vfmd link (Yukai Chou).
• Link to babelmark2 legacy page.
• Use https scheme in spec (including examples) (#751, Yukai Chou).
• Fix unicode symbols example in emphasis syntax (rhysd).
• Remove confusing "first" before "link label."
• Add symbols to unicode punctuation (Titus Wormer).
• Update link to AsciiDoc (Jarno Elovirta).
• Add search element to list of known block elements (Titus Wormer).
• Terminology: compact -> collapsed reference link.
• Correct emphasis typo (Mike Pennisi).
• Remove restrictive limitation on inline comments; now we match
  the HTML spec (Titus Wormer).
• Fix spelling of "down side" to "downside" (Erik Duveblad).
• Fix typo (missing words) (Denis Howe).
• Remove source element as HTML block start condition (Lukas Spieß).
• Fix typo (Nicolas Hedger).
• spec_tests.py improvements (Martin Fischer): introduce --track option,
  simplify code, show diffs of normalized HTML, fix UnicodeDecodeError catching.
• Use archived gmane links.
• Fix typo (#693, Salim B).
• Remove unnecessary lines from #95.
• Fix typo (#683, nadashin).