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