(feat) Chips: validation for chips

#### Bug, feature request, or proposal: feature

#### What is the expected behavior?
Custom validators and `mat-error` should work when placing a chip-list inside a mat-form-field
There should be a two-fold validation: if there is a nested input for adding new chips, the input should validate entry previous to adding it.
The data of the whole chip-list should be validated.
It should work seamlessly with ngModel and validation api.

#### What is the current behavior?
The nested input cant be validated.
The form-field is bound to the chip-list control state, but the chip-list's `value` doesn't hold the whole array, but just the selected item.
It doesn't react to addition/insertions, so no validation can be trigger/shown

#### What are the steps to reproduce?
Providing a StackBlitz/Plunker (or similar) is the *best* way to get the team to see your issue.

Plunker starter (using on `@master`): https://goo.gl/uDmqyY

StackBlitz starter (using latest `npm` release): https://goo.gl/wwnhMV

#### What is the use-case or motivation for changing an existing behavior?
Use the chip-list control to edit array data in a form, and properly validate the input

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

#### Is there anything else we should know?

Inizia dalla riproduzione collegata di Plunker o StackBlitz e traccia la mat-chip-list all’interno di mat-form-field, includendo l’input annidato e il binding del form. Il lavoro è completo quando i validator personalizzati e mat-error funzionano sia per l’inserimento dei chip sia per l’intera chip-list, con le aggiunte riflesse tramite ngModel e l’API di validazione.

Scritto dal modello di indicizzazione a partire dal testo della issue.