Make options argument optional on initialization

Code below will fail because we didn't pass an empty object on initialization, which will result in undefined variable on the first assignment `this.el = opts.el`.

```js
var AwesomeFormView = FormView.extend({
    fields: [
        new SelectView({
            label: 'Pick a color!',
            name: 'color',
            value: 'blue',
            options: ['blue', 'orange', 'red']
        })
    ]
});

var awesomeFormView = new AwesomeFormView();
```