Store Hooks objects

In some apps I store want the finals hooks (like post_controller or post_system) to execute an action based on the first hooks. So I write a Hook class and set properties to do this, but the CI initializes a new hook object for every call. This edition makes hook objects storable. When I use this definition in the config/hooks.php

```php
$hook['trigger'] = array(
    'class' => 'class',
    'function' => 'function',
    ...
    'store_object' => TRUE
);
```

The same object will execute all hooks for this class.