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
Ideally, Twig.js should pass the same tests as the PHP version. I'd prefer that we port the PHP tests writing our own only when we notice an omission in the PHP tests.

That being said, I think that your tests are adequate in this case, and I also have no say in the matter :-)