Create an arc todo workflow for quickly creating a task.

Summary:
Run arc todo o rly? to create a Maniphest task
titled 'o rly?'. Pass --cc to add CCs besides yourself.

Test Plan: Ran arc todo o rly?. Behaved as expected.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T749, T1325

Differential Revision: https://secure.phabricator.com/D2952