How to enable the twig truncate filter (Text extension) in Symfony2 beta5

Problem ?

In previous versions of Symfony2, to enable a extension you needed to add the Text and/or Debug extensions to an "extensions" array in your config.yml

//app/config/config.yml
# Twig Configuration
twig:
 debug:            %kernel.debug%
 strict_variables: %kernel.debug%
 extensions:
     - twig.extension.debug
     - twig.extension.text

This was because the extensions were added to the twig.xml file

//vendor/symfony/src/Symfony/Bundle/TwigBundle/Resources/config/twig.xml
...
  <service id="twig.extension.text" class="Twig_Extensions_Extension_Text" public="false">

     <service id="twig.extension.debug" class="Twig_Extensions_Extension_Debug" public="false">
...

But, this is the way to do it now :

// app/config/config.yml
...
services:
 twig.extension.text:
     class: Twig_Extensions_Extension_Text
     tags:
         - { name: twig.extension }

I spent some time trying to figure out how to do it, so I hope it helps someone.