Fix some issues with code blocks and ASCII art

Summary:
Ref T3558. We currently trim() blocks, but should just rtrim() them, so we don't lose extra indentation on the first line.

Also, be more liberal about parsing trailing space after triple-backtick.

Test Plan: Unit tests.

Reviewers: btrahan, garoevans

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T3558

Differential Revision: https://secure.phabricator.com/D6481