Take blocks into account into Diviner highlighter

Summary: This is still far from ideal but at least solves the pretty common use of code block with C++-style comments in Remarkup.

Test Plan:

lang=remarkup
a//b//

  a // a
  b // c

//d
e//

//f

g//

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5094