Declarative formatting

Allows declarative, inline markup to be used as select2 formatting functions.

```html
<ui-select2 options="{allowClear:true,multiple:true,query:fuzzyMatchQuery}" ng-model="bestIslands">
  <format-result>
   <img src="pirate_flag.png" />{{ text }}
  </format-result>
</ui-select2>
````

The [specs](https://github.com/duncanbeevers/ui-select2/blob/08336b10cf7d9fa582318eb9943eda086556fa18/test/select2Spec.js#L216-L324) cover all the functionality added, and the demo page [has been updated with interactive examples](https://github.com/duncanbeevers/ui-select2/blob/08336b10cf7d9fa582318eb9943eda086556fa18/demo/index.html#L224-L404).