R1:bb58a123e633

Modularize Celerity postprocessors

Summary: Not sure if we want this, but it seems to work fine.

Test Plan: {F516736}

Reviewers: joshuaspence, chad

Reviewed By: joshuaspence, chad

Subscribers: joshuaspence, epriestley

Differential Revision: https://secure.phabricator.com/D13363