Add truncate filter

 - port truncate filter from twig extention repo https://github.com/twigphp/Twig-extensions/blob/master/lib/Twig/Extensions/Extension/Text.php#L42
-  add unit test
-  build files
The test cases for the PHP version are as follows:

```php
public function getTruncateTestData()
    {
        return array(
            array('This is a very long sentence.', 2, false, '...', 'Th...'),
            array('This is a very long sentence.', 6, false, '...', 'This i...'),
            array('This is a very long sentence.', 2, true, '...', 'This...'),
            array('This is a very long sentence.', 2, true, '[...]', 'This[...]'),
            array('This is a very long sentence.', 23, false, '...', 'This is a very long sen...'),
            array('This is a very long sentence.', 23, true, '...', 'This is a very long sentence.'),
        );
    }
```