Add support to nested variables

I have to do more tests, but I wanna see what you think about the idea to have nested variables.

**file.ini**
````ini
db.driver = "mysql"
db.host   = "localhost"
db.name   = "my_database"
db.dsn    = "[db.driver]:host=[db.host];dbname=[db.name]"
````

**file.php**
````php
$container = new Container('file.ini');
var_export($container->db);
````

**output**
````
array(
    'driver' => 'mysql',
    'host' => 'localhost',
    'name' => 'my_database',
    'dsn' => 'mysql:host=localhost;dbname=my_database',
)
````