md-select: add compareWith function

Currently, md-select values are compared by object reference. This is a bit inflexible. We want to allow people to compare options by a custom comparator in case they only care about the object’s ID or some other property.

Example markup:

<md-select placeholder="Food" [(ngModel)]="food" [compareWith]="compareById">
   <md-option *ngFor="let food of foods" [value]="food"> {{ food.name }} </md-option>
</md-select>

compareById(obj1, obj2) {
   return obj1.id === obj2.id;
}

This will be parallel to functionality being added to native selects in core.

[linuradu]

Sounds good, regarding commit 😃 When will be the next release?

[reppners]

Desperately need this 👍