Add support for Expressions within templates

[mlabrum]

This adds support for using symfony Expressions.

This class is pretty much a copy of https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Extension/ExpressionExtension.php

This allows you to use expressions with the security extension http://symfony.com/doc/current/book/security.html#book-security-template-expression

{if '"ROLE_ADMIN" in roles or (user and user.isSuperAdmin())'|expression|is_granted }
Hi Admin!
{/if}

[vitorbrandao]

Merged. Thank you @mlabrum!

[vitorbrandao]

Added an example to the documentation: https://smartybundle.readthedocs.org/en/latest/extensions.html#complex-access-controls-with-expressions