commonmark.js 0.31.0

• Update to 0.31 spec.txt.
• Treat unicode symbols like punctuation for purposes of flankingness.
  This updates the library to conform to the 0.31 spec.
• Do not process &-entities that don't end in ; (#278, Michael Howell).
• Html renderer: don't add language- to code block class
  if the info string already starts with language- (#277).
• Fix pathological regex for HTML comments (#273).
• Track underscore bottom separately mod 3, like asterisk (Michael Howell).
• Fix list tightness (taku0).
• Fix "CommomMark" typo (#270, Martin Geisler).
• Declarations do not need a space, per the spec (commonmark/cmark#456).
• Allow <!doctype to be case-insensitive.
• Fix HTML comment regex.
• Fix HTML comment regex to conform to commonmark/commonmark-spec#713.
• Fix a pathological performance case in link titles.
  Thanks to Stefan Dobre for reporting.
• Allow closing fence to be followed by tabs (#258).
• Replace deprecated String.prototype.substr() (#254, CommanderRoot).
• Fix bug with reference link normalization. We were only collapsing the
  first group of consecutive whitespace, not following ones.