add possibility to dump all bundle configurations in ConfigDumpReference

In PhpStorm Symfony2 plugin we now have the possible to autocomplete config / security keys. It uses the xml output of `config:dump-reference`. plugin includes a default dump on standard edition, but per project dump is possible `/idea/symfony2-config.xml`.

Currently there so no way to dump all configs. would be nice if everyone can create it on a single command:

```php
php bin/console config:dump-reference * --format=xml
```

corresponding foreign file
https://github.com/Haehnchen/idea-php-symfony2-plugin/blob/master/src/resources/symfony2-config.xml

![phpstorm-symfony2-plugin](https://cloud.githubusercontent.com/assets/1011712/3076794/aa639a9a-e3fd-11e3-8b8e-26e8691fd2fc.png)


it's wrong to use the extension alias as the element name. It should be `<config>`.