Allow an unassigned flag to be passed to `arc tasks`

Summary: Returns unassigned tasks

Test Plan: Run `arc tasks`, `arc tasks --unassigned`, make sure I get the correct results

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3201

Differential Revision: https://secure.phabricator.com/D6009