Adding support for custom additional functions and filters

Functions and filters can be added by config. Now you can add functions and filters as key&value pair:
```   
'functions' => array(
        'some_function' => array('Some_Class', 'some_method'),  // Type Callable
),
'filters' => array(
        'some_filter' => array('Some_Class', 'some_method'), // Type Callable
),
```

And use them in template:
```
{{ some_function() }}
{{ 'Text'|some_filter }}
```