Make arc tasks print output in a unicode-aware and console-width-aware way

Summary:
Currently, we print arc tasks in a console-agnostic and unicode-unaware way, so:

• Tasks with multibyte characters get aligned incorrectly (see T1831); and
• narrow terminals plus long task names results in a broken display.

Test Plan: Ran arc tasks. Will post screenshots.

Reviewers: btrahan, vrana, Korvin

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T749, T1831

Differential Revision: https://secure.phabricator.com/D3568