Add pygmentize support for Puppet files.

Summary: A Puppet lexer was added to pygmentize in v1.6rc1. Apply the Puppet lexer to *.pp files.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9318