Allow callbacks as 'function' and 'filter'

I've found similar pull-request https://github.com/yiiext/twig-renderer/pull/13 , but it looks like it could be achived by much simpler approach.
This pull-request adds what it should: allows passing callbacks as `function` and `filter`. Also it fixes misbehavior (misunderstanding?) of working with `options` - they are not required, so we should not rely on them heavily, just pass them through if they are present.