Add a --browse flag to arc diff, to open objects in a browser after creating them

Summary: Ref T5781. Add a flag to optionally open a web browser after creating a diff or revision.

Test Plan: Created this revision with arc diff --browse !!!

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Maniphest Tasks: T5781

Differential Revision: https://secure.phabricator.com/D10141