[proposal][wip] Helper configuration from application

Sample configuration with this approach:

```php
return array(
	// ...
	
	'components' => array(
		// ...
		
		'db' => array(
			// ...
		),
		
		// ...
	),
	
	// ...
	
	'helpers' => array(
		'Html' => array(
			'closeVoidElements' => true,
		),
	),
);
```

Solves the problem described by @samdark here: https://github.com/yiisoft/yii2/pull/344#issuecomment-18276413