feat(input): interpolates form input name attrs for ngModel

@frankhuster / @niargh has brought up an issue with using the `$index` of a repeated element for the name attribute of a form input.

I don't know how common of a use-case it is, but it seems not especially harmful to do a one-time interpolation in post-link.

It shouldn't be a breaking change unless people are using interpolation markers in their names.
What is this case for? radio buttons? Please add a test that shows the usecase.
The comment says that this creates 2 references to the same control, but it does not say how the other reference will get cleaned up if the control is removed (e.g. if it is part of `ng-if` or `ng-repeat`).