Fix Remarkup list styles

Summary:

• Don't throw away data.
• Accept item delimiters like "--", "###" etc.
• Use number of delimiters to control indent level if it's 2+, otherwise use

space indentation like Markdown.

Test Plan: Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1660