Bug1068860

Since we're updating an API of a shared class. How about also adding ability to pass l10n args at the same time.
The API would work then this way:
 - if string -> setAttribute('data-l10n-id', message');
 - if object {id: string, args: {}} -> mozL10n.setAttributes(node, id, args);
 - if object {raw: string} -> textContent = message;

We could then standardize this approach with other APIs and make it easier for devs to learn how to pass the right one.