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.
This can be simplified to:
```
if (form[oldName] === control) {
  delete form[oldName];
}
form[newName] = control;
control.$name = newName;
```