Support an 'editor' config in arc

Summary: See D2955. Allow "arc set-config editor ..." to override all other editor settings.

Test Plan: Ran "arc set-config editor 'mate -w'", am typing this in textmate.

Reviewers: btrahan, ezfoxie

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1309

Differential Revision: https://secure.phabricator.com/D2956