feat(ngRepeat): add count option to repeater expression

`ngRepeat` expressions can now accept an optional count varaiable:

    count for item in array
    count for (key, value) in object

The count is a user-defined expression assigned to the `ngRepeat`'s
parent scope. An example of intended use:

    <input type="text" ng-model="search">
    <div>Showing {{itemCount}} of {{items.length}}</div>
    <ul>
      <li ng-repeat="itemCount for item in items | filter:search">
        {{item.name}}
      </li>
    </ul>