Callables in template

Hey people,

We needed to add closures to the templates in Isotope and as this is quite a cool feature we thought we might also add this to the Contao 3.2 core.

You can then do things like this:
```php
$objTemplate->sum = function($a, $b) {
    return $a + $b;
}
```

So your users can use that function in the template:
```html
<p>The sum is: <?php echo $this->sum(3, 7); ?></p>
```