• Changed handling of tabs. Instead of having a preprocessing step
  where tabs are converted to spaces, we now handle tabs directly in
  the parser. This allows tabs to be retained in code blocks and code
  spans. This change adds some general language to the effect that,
  for purposes of determining block structure, tabs are to be treated
  just like equivalent spaces.
• Completely rewrote spec for HTML blocks. The new spec provides
  better handling of tags like <del>, which can be either block
  or inline level content, better handling of custom tags, and
  better handling of verbatim contexts like <pre>, comments,
  and <script>.
• Added 9-digit limit on ordered list start number.
  Some browsers use signed 32-bit integers for indexing
  the items of an ordered list, and this can lead to odd
  or undefined behavior if 10-digit start numbers are allowed.
• Allow (non-initial) hyphens in tag names (#239). Custom
  tags in HTML5 must contain hyphens.
• Clarified that HTML block is closed at end of containing
  block, not just end of the document (as with fenced code blocks.)
• Specify nested link definition behavior in prose (Benjamin
  Dumke-von der Ehe).
• Added test for edge case in link reference parsing
  (Benjamin Dumke-von der Ehe, see commonmark/commonmark.js#49).
• Added link tests with fragment identifiers and queries (David
  Green, #342).
• Added test cases with a literal backslash in a link destination
  (see commonmark/commonmark.js#45).
• Added test for entity ≧̸ which resolves to two code points.
  Put entity tests on several lines for readability (see
  commonmark/commonmark.js#47).
• Fixed broken "pre" literal HTML example. Contents
  weren't escaped properly.
• Simplified defn of "unicode whitespace character,"
  rectifying omission of line tabulation, U+000B (#343).
• Removed ambiguity in definition of "line" (#346).
• Rewrapped two prose lines so + does not begin a line (#347).
• Added another test with overlapping emphasis markers.
• Fixed link to 'attributes'.
• Revised appendix, "A parsing strategy," and
  added a description of emphasis/link parsing algorithm.
• spec_tests.py - set options for conversions, set library
  paths in a more cross-platform way.
• spec_tests.py: force utf-8 on test failure output and
  --dump-tests (#344, #345).
• spec_tests.py: Properly handle visible tab → in expected output.
• normalize.py: Don't collapse whitespace inside pre tag.
• Added spec.html to .gitignore (#339).
• Add -dev suffix to spec version after release (eksperimental).
• Rename "non-space" to "non-whitespace" (Konstantin Zudov, #337).