Add a pregsprintf function

Summary: See rP14507dc64bf42373e627a90016cd63f06e82e19b.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12310