Add Pxxx to phabot.

Summary:
Summarize (or at least link to) pastes when P<id> is given in a channel. This depends on D552!

Test Plan:
04:34:03 <@codeblock-phabot> P15 - http://phabricator.local/P15
04:35:22 <@CodeBlock> P1 asdf foo Bar Baz Diet Sprite P4
04:35:24 <@codeblock-phabot> P1 - http://phabricator.local/P1
04:35:24 <@codeblock-phabot> P4 - http://phabricator.local/P4

Reviewers:
epriestley

CC:

Differential Revision: 553