Add support for escaping characters with the date filter

This PR adds character escape support, making the date filter closer to "PHP-style".

PHP usage being modelled is shown at http://us3.php.net/manual/en/function.date.php#example-2306

Per my and @paularmstrong preference, this PR supports one-escape-needed-per-character. As Paul says:
> One escape per character makes more sense to me, as there doesn't seem to be a way to un-escape without a space.