Read default values of custom config options

Summary: Because the Default configuration provider is loaded before custom libraries, any config options specified in them don't get a default values.

Test Plan: Looked at /config/

Reviewers: epriestley, codeblock, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4532