Pass store manager's response to editor's store callback

Hi @artf, We have a need to get a response of the store manager when calling editor.store() method with a callback (we return generated template id in this case from the server). I'm pretty sure you have it implemented for editor.load() method. I think the change is pretty simple in /src/editor/model/Editor.js Let me know if you want me to do a pull request. Thank you.