Allow PhutilCommandString to escape as Powershell
Summary: This allows PhutilCommandString to escape as Powershell.
Test Plan: Unit tests
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D10249