Make "cmd flow --help" work like "cmd help flow"
Summary: Currently, the "--help" flag forces us to generic help, but in the "
Test Plan: Ran cmd, cmd help, cmd --help, cmd flow --help, cmd help flow, cmd help --help.
Reviewers: davidreuss
Reviewed By: davidreuss
CC: aran
Differential Revision: https://secure.phabricator.com/D4106