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