Preserve whitespace more faithfully when processing %%% remarkup blocks
This improves the output for some edge cases with %%% blocks. Notably, blocks did not emit a block-level spacing tag, so this:
...rendered on a single line.
Added unit tests.
Reviewed By: btrahan
Subscribers: chasemp, epriestley
Differential Revision: https://secure.phabricator.com/D10984