Add %LR support to csprintf.

Summary: Allow csprintf to format an array of readable strings.

Test Plan:
I didn't create any test cases because there aren't any existing test cases for csprintf. Instead, I used the following script:

<?php

require_once __DIR__.'/scripts/__init_script__.php';

echo csprintf('%LR', array('foo', '--not', 'bar'))."\n";
echo csprintf('%LR', array('foo', '--not bar'))."\n";

The output was as follows:

foo --not bar
foo '--not bar'

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9229