Use more liberal whitespae detection for `%%%` and `---` remarkup rules

Summary: This relaxes detection of whitespace for literal blocks and horizontal lines. In particular, when these rules are quoted, they appear as, e.g., `> ---`, which currently does not show a quoted horizontal line.

Test Plan: Added and ran unit tests. Quoted literal and HR rules.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10972