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:

```lang=php
<?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