Improve text mode rendering of reverse Remarkup list staircases

Summary: Ref T5102. The text-only mode of these was a bit silly.

Test Plan: Added and executed unit tests.

Reviewers: asherkin, btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5102

Differential Revision: https://secure.phabricator.com/D10044