Add "and" support to "ref"

Summary: Fixes T8038. Allow PhabricatorCustomFieldMonogramParser to handle "and". This means that Ref Tx, Ty and Tz will correctly return array('monograms' => array('Tx', 'Ty', 'Tz').

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Maniphest Tasks: T2, T3, T1, T8038

Differential Revision: https://secure.phabricator.com/D12682