add twig text_extension

I saw this in some bundles
```
{{ object.str | length > 40 ? object.str | slice(0, 40) ~ '...' :object.str }} 
```

It may be usefull to load twig text extension ( http://twig.sensiolabs.org/doc/extensions/text.html ) in the core in order to get this (and beneficiate everywhere):
```
{{ object.str | truncate(30) }}
```