Private Message User IRC Command

Summary:
Added phabot irc command to directly message a user rather than outputting in a
channel.

Syntax:

ex:

`Korvin, D1717

results in phabot private messaging me the info on D1717

Test Plan: nick, [DTPVF]n

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1717