Parse markup for external interpreter

Summary:
This should pick on any interpreters extending `PhutilRemarkupBlockInterpreter` after matching a specific syntax, see task.

Fixes T3602

Test Plan: unit test

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3602

Differential Revision: https://secure.phabricator.com/D7012