Add a --browse flag to `arc todo`

Summary: Ref T5781. Like D10141, but for `arc todo`. Fixes T5787.

Test Plan: See T5787.

Reviewers: btrahan, csilvers

Reviewed By: csilvers

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781, T5787

Differential Revision: https://secure.phabricator.com/D10142