Support arbitrary start numbers for ordered lists in remarkup

Summary: Fixes T6377.

Test Plan: Unit tests, mostly. Also checked a local install to see if this felt sensible.

Reviewers: Thirtyate, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6377

Differential Revision: https://secure.phabricator.com/D10742