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