feat($compile): support directive virtual groups

By appending  X-start and X-end to a directive it is now possible
to have the directive act on a group of elements.

It is now possible to iterate over multiple elements like so:

```
<table>
  <tr ng-repeat-start="item in list">I get repeated</tr>
  <tr ng-repeat-end>I also get repeated</tr>
</table>
```