Makes attribute arguments passed to a component two way binding

If you have a "master" component with a scope value like:

```js
scope: {
  visible: true
}
```

Pass that property to a low-level compute like:

```html
<low-level showing='visible'></low-level>
```

If low-level has code like

```js
this.attr('showing', true)
```

it should change the parent scope's `visible` property.