commonmark.js 0.28.0

• Update spec to 0.28.

• Align punctuation regex with spec (#121). Previously some ASCII
  punctuation characters were not being counted, so ^_test_ came out
  without emphasis, for example.

• Simplified a logical test, making it closer to the wording of the spec.

• Don't parse reference def if last ] is escaped (#468).
  E.g.

  [\ ]
  
  [\]: test
  

• Dingus Makefile: remove ref to obsolete html.js.

• Removed obsolete lib/xml.js (replaced by lib/render/xml.js).

• Allow tabs before and after ATX closing header (Erik Edrosa).

• Change precedence of Strong/Emph when both nestings possible.
  This accommodates the spec change to rule 14.
  Note that commonmark.js was not previously in conformity
  with rule 14 for things like ***hi****.

• Calculate "mulitple of 3" for delim runs based on original number
  of delims, not the number remaining after some have been
  used.

• Make esc() method abstract and overridable (muji).

• README: update documentation for overriding softbreak and esc (#118).

• Remove old XMLRenderer implementation (muji).

• package.json: use shorter form for repository.

• Don't export version in lib/index.js.
  Instead, users can get version from package.json:
  require('commonmark/package.json').version.

• Removed remnants of old html renderer (#113).
  Now we use lib/renderer/html.js.

• Hand-rolled parser for link destinations.
  This allows nested parens, as now required by the spec.

• Fix regression test example (Colin O'Dell).

• dingus: Fixed iframe on load.