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
Be more liberal about whitespace in the "interpreter block" rule

Summary: Currently, we require `x (y) {{{z}}}`. Permit less whitespace, `x(y){{{z}}}`.

Test Plan: Unit test.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7338