feat(select): match options by expression other than object identity

Extend ng-options with a new clause, "track by [trackByExpression]", which can be used when
working with objects.  The `trackByExpression` should uniquely identify select options objects.
This solves the problem of previously having to match ng-options objects by identity.
You can now write: `ng-options="obj as obj.name for obj in objects track by obj.id"`
The "track by" expression will be used when checking for equality of objects.

Examples:
<select
    ng-model="user.favMovieStub"
    ng-options="movie as movie.name for movie in movies track by movie.id">
</select>

scope: {
  user: { name: 'Test user', favMovieStub: { id: 1, name: 'Starwars' } }
  movies: [{ id: 1, name: 'Starwars', rating: 5, ... }, { id: 13, ... }]
}

The select input will match user favMovieStub to the first movie in the movies array, and show
"Star Wars" as the selected item.

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