Allow real names in addition to usernames for --cc, not just --rr

Summary:
I also allow a trailing @ to turn off the real-name lookup.

Test Plan:
When I send this for review, I will do
   arc mail --rr chris@ --cc ben
and make sure I get the chooser to pick a ben.

Reviewers: chris
Reviewed By: chris
CC: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2844