Enable the possibility to extends the twig environment

Q | A
------------ | -------------
Bug fix? | no
New feature? | yes
BC breaks? | no
Deprecations? | no
Fixed tickets | #208

This PR allow people who extends the generator for custom use to alter the twig environment. 

For example, it will allow to add an extension :
```php
class MyGenerator extends Generator 
{
   protected function getTwigEnvironment(){
      $twig = parent::getTwigEnvironment();
      $twig->addExtension( new MyTwigExtension() );
      return $twig;
   }
}