R1:4ea9d76f66f1

Add some missing type hints

Summary: Add some typehints for Remarkup rules.

Test Plan: Browsed around some Remarkup text.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12694