Support for extending array definitions

Attempt at implementing #214 for arrays.

```php
return array(
    'array1' => array(
        'value',
    ),
    'array2' => array(
        'value',
    ),
);
```

Extensions:

```php
return array(
    'array1' => DI\extend()->add(array(
        'another value',
    )),
    'foo-bar' => DI\extend('array2')->add(array(
        'another value',
    )),
);
```