Add {{boundTemplate}} helper

It's simply the {{template}} helper, that's receive a path to templateName, instead of templateName.
For example it can be used it for switch between present mode and edit mode for implement inline edit of model's attribute:

```coffeescript
EditableBase = Ember.View.extend
  templateName:  'editable/base'
  isEditorState: false

EditableString = EditableBase.extend
  presentTemplateName: 'editable/string_present'
  editTemplateName: 'editable/string_edit'
```
editable/base.handlebars:
 ```handlebars  
{{#if isEditorState}}
  {{boundTemplate 'editTemplateName'}}
{{else}}
  {{boundTemplate 'presentTemplateName'}}
{{/if}}
```