Feature : improved extends

Note: please merge feature/new_requires_key before since this PR suppose that this has been done before (they are dependent)

This small feature doesn't change anything on standard case, but allows an application to extend an other without merging configuration files (in this case only the bootstrap is loaded). Often the recursive merge aren't enough and handy : therefore it can be useful to disable this behaviour.

instead of writing :

```php
'extends' => 'app'
```

We can write

```php
'extends' => array(
    'application' => 'app',
    'extend_configuration' => false // default to true
)
```