Allow specifying a name property to {{input}}.

This is quite helpful when using the `ember-testing` helpers with jQuery selectors. Take the following example:


```handlebars
{{#form-for controller}}
  {{input firstName name="first-name"}}
{{/form-for}}
```

```javascript
visit('/');
fillIn('input[name="first-name"]', 'Robert');
```

Ultimately, we could infer this value based on the first argument (the field name) and add a default value all the time.