add a 2nd bound property so we can build DOM attribute correctly. 

Since Ember's default method of creating DOM elements with boolean attributes is to repeat the attribute name as the value, we need a 2nd property to return a strict `true` or `false` for the `contenteditable` binding. 

The syntax for using this in a template now becomes

```
{{view App.ContenteditableView
            valueBinding="description"
            tagName="p"
            editableBinding="somePropertyOnController"}}
}}
```