Add --skip-arcconfig option

Summary:
Continuation of D4732 - when we don't care about loading an arcconfig,
allow that to be specified.

Test Plan: chmod -r .arcconfig; bin/arc help --skip-arcconfig

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4750