Cors: check origin against a list of allowed hosts

For CORS I wanted to be able to check the request origin on a list of allowed hosts.

I implemented it like this : 
```
$app->add(new \SlimJson\Middleware(array(
	'json.cors' => function($requestOrigin) {
		
		$allowed = array(
			'http://localhost:9000',
			'http://www.domain.com',
			'http://domain.com'
		);

		if(in_array($requestOrigin, $allowed)) {
			return $requestOrigin;
		} else {
			return false;
		}
	},
```
Now when the requestOrigin is in the list of allowed hosts it returns the request origin and shows the 'Access-Control-Allow-Origin' header. If it is not in the allowed hosts list it returns false and skips setting the 'Access-Control-Allow-Origin' header.

This change should be backwards compatible with the old json.cors config.