feat($compile): support multi-element directive

By appending  directive-start and directive-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>