Support processing Remarkup in bulk with remarkup.processbulk Conduit method

Summary: This adds support for a Conduit method to process Remarkup content in bulk. It also updates the getEngineContexts methods to support any missing contexts.

Test Plan: Ran the command and processed a few sets of text.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4046

Differential Revision: https://secure.phabricator.com/D7479