feat(forms): add support for standalone ngModel dirs inside forms

[kara]

This PR allows the use of ngModels within forms without necessarily requiring the registration of their form controls with the parent form.

r: @vsavkin
cc: @StephenFluin: can you look at the new error message?

Before
If you wanted to include an ngModel within a form tag, it had to have a name attribute so that the ngModel could be registered with the parent form properly (for aggregate form validation and value).

<form>
    <input [(ngModel)]="person.name" name="first">
    <input [(ngModel)]="person.food">      //  always throws error because name is not set
</form>

However, in some rare cases, it might be desirable to have an ngModel within a form tag, but not registered with the form. This setup was not supported and would throw an error.

After

<form #f="ngForm">
    <input [(ngModel)]="person.name" name="first">
    <input [(ngModel)]="person.food" [ngModelOptions]="{standalone: true}"> 
</form>

{{ f.value | json }}       // {first: ""}

Now, if for some reason you need to have a standalone ngModel within a form tag, it's still usable without registering it with the parent form by setting the standalone property in ngModelOptions. This way, you can explicitly dictate that you don't want the form registration, but users who may forget the name attribute will still get a helpful error.

It's worth noting that ngModels can still, as always, be used outside of a form tag without a name attribute or an ngModelOptions property.

[vsavkin]

should it public or private?

[kara]

Probably private; I'll update.

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