Markdown reader: revised definition list syntax (closes #1429).

* This change brings pandoc's definition list syntax into alignment
  with that used in PHP markdown extra and multimarkdown (with the
  exception that pandoc is more flexible about the definition markers,
  allowing tildes as well as colons).

* Lazily wrapped definitions are now allowed; blank space is required
  between list items; and the space before definition is used to
  determine whether it is a paragraph or a "plain" element.

* For backwards compatibility, a new extension,
  `compact_definition_lists`, has been added that restores the behavior
  of pandoc 1.12.x, allowing tight definition lists with no blank space
  between items, and disallowing lazy wrapping.