• Added several more tab-handling cases (see commonmark/cmark#101).
• Fixed spec test for tabs. In the blockquote with a tab following
  the >, only one space should be consumed, yielding two spaces
  at the beginning of the content.
• Update license year range to 2016 (Prayag Verma).
• Fixed typo: setext heading line -> setext heading underline (#389).
• Fixed date 2015->2016 (#388)