Expand Remarkup interpreter block capabilities a little bit

Summary:
Ref T3274. Ref T3591. Ref T3964. Allows interpreters to access the engine so they can check for text mode, grab the viewer/config, etc.

Also, make the unit tests not depend on trailing whitespace to pass.

Test Plan: See next revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3274, T3591, T3964

Differential Revision: https://secure.phabricator.com/D7331