Add a custom due date field in Maniphest

Summary:
Based on instructions from
http://www.phabricator.com/docs/phabricator/article/Maniphest_User_Guide_Adding_Custom_Fields.html

Test Plan:

Auditors: csilvers

CC:
Update khan.conf.php

Fair enough!
Merge pull request #1 from jnetterf/master

Add a custom due date field in Maniphest