[button-toggle-group] bound to NgModel throws error if multiple selection is used

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When <button-toggle-group multiple [(ngModel)]="property"> is used, the model property should be updated with a list (object or array) of values selected in the toggle group.

Alternatively, improve the docs with a note that the ngModel binding is not supported in multiple selection mode.

What is the current behavior?

The app crashes with error: No value accessor for form control. The same code works when group is in exclusive selection mode

What are the steps to reproduce?

See this plunker. The app will not run unless you remove the multiple attribute from the toggle group in /app/app.component

What is the use-case or motivation for changing an existing behavior?

It’s a bug

Which versions of Angular, Material, OS, browsers are affected?

Angular 2 rc.5, Material 2 alpha 7-2

[devversion]

This seems to be intentionally while looking at the source.

I think it would be probably good to support ngModel for multiple mode as well, and emit an array of the selected md-button-toggle’s to the ngModel.

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