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