Randomsauce - make the align-rightness of PhabricatorDropdownMenu configurable

Summary: using this for the Conpherence drop down menu and it should be left aligned. This makes that work.

Test Plan: using it in conpherence and it aligns how I want it. also still aligns right in other existing usage spot - differential

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Differential Revision: https://secure.phabricator.com/D6002