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
``truncate`` is part of the optional text extension, not base Twig. I'm not sure what's the policy on adding this functionality?

I saw that ``30`` is the default length in the PHP implementation.

This looks sane, I did not test it.