R1:7863956746be

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