commonmark.js 0.23.0

• [API change] Rename nodes: "Html" -> "HtmlInline" (#63).
• [API change] Add CustomBlock, CustomInline.
• [API change] Rename "HorizontalRule" -> "ThematicBreak".
• [API change] Rename "Header" -> "Heading" (spec change).
• Don't allow whitespace between link text and link label
  of a reference link (spec change.)
• Fixed calculation of list offsets (#59).
• Allow tab after bullet list marker (#59).
• advanceOffset - copy the code from libcmark.
• Fixed a list/tab/padding corner case (#59).
• Escape HTML contents in xml output, as the DTD requires PCDATA.
• xml renderer - added xmlns attribute (commonmark/commonmark-spec#87).
• Test on node.js 5.x and 4.x (Nik Nyby). Remove testing on iojs.
• Initialize _listData to {} when creating Node (#74).
• Added version check for uglify; updated dist files (#69).
• Fix typo in breakOutOfLists description (Nik Nyby).
• Updated benchmarks in README.md.