Use different numbering schemes for nested ol's

Summary:
Nested ol's using the same numbering scheme are hard to parse visually.

This just makes it a little nicer.

Maybe we could add greek too...

Test Plan:

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Projects: Remarkup

Maniphest Tasks: T7976

Differential Revision: https://secure.phabricator.com/D12311