Display informative message when arc launches an editor

Summary:
Update PhutilInteractiveEditor to allow specifying a "task message" which will be displayed just prior to launching the user's editor.

Refs T3271

Test Plan: I ran several arc diff commands in varying states to invoke my editor and verified that it printed out the text indicating that my editor was being launched.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T3271

Differential Revision: https://secure.phabricator.com/D21700