Allow extra options for helpers

As proposed in https://github.com/rcrowe/TwigBridge/issues/68

Allows Twig_SimpleFilter/Function, or an array with either just the
options, or a callback. Without callback, is just calls that function.

'link_to' => array(
'is_safe' => array('html')
),
'custom_function' => array(
'is_safe' => array('html'),
'callback' => function(){ return '..'; }
),