Arcanst - add support for custom statuses to arc close workflow

Summary:
Fixes T4938. Now that we have customs statuses, this workflow needs to know about 'em.

One fun caveat is that Conduit isn't up and running to generate the help method, so I had to add a parameter to have the statuses list out. This could maybe be omitted since entering erroneous status options does indeed tell you the correct options.

Test Plan:
arc close --list-status -- got a list of statuses
arc close T1 -- closed T1
arc close T1 -- error T1 closed already
arc close T1 -s foobar -- T1 status set to foobar

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1, T4938

Differential Revision: https://secure.phabricator.com/D8938