Markdown: accept GitHub style -- for <hr>

GitHub flavor markdown apparently accepts -- surrounded by
blank lines as an <hr> in HTML:

  Blah blah lots of text.

  --

  More text after the rule.

Support this by adding a custom rule to the parser that
recognizes this style and creates a HRule node.

Change-Id: I406e6d9def04ff2e3589a72316c28b3ddc773cd0