withHash helper to create a new context from the attribute hash

Hello!

Sometimes I need to rename keys of the current context or create a new context. So, I wrote a little helper to this.

```html
{{#withHash subject="Feedback" message=this.text}}
    <h1>{{subject}}</h1>
    <div>{{message}}</div>
{{/withHash}}
```

Hope it will be useful to somebody else.