Support multiple environments in the same build

> Please provide us with the following information:
> ---------------------------------------------------------------

### OS?
> Mac OSX (Sierra)

### Versions.
```
angular-cli: 1.0.0-beta.24
node: 6.9.2
os: darwin x64
@angular/common: 2.4.1
@angular/compiler: 2.4.1
@angular/core: 2.4.1
@angular/forms: 2.4.1
@angular/http: 2.4.1
@angular/platform-browser: 2.4.1
@angular/platform-browser-dynamic: 2.4.1
@angular/router: 3.4.1
@angular/compiler-cli: 2.4.1
```

### This is a feature request

I started internationalizing my application, and I met the following problem: when generating the bundle for the French locale (for example), I should include the locale-specific fr.js script of moment.js. Other libraries could also provide locale-specific JS files, or could need code that is specific to that locale (to internationalize a datepicker, for example).

I think the best way to do that would be to create an 'fr' environment file, simply containing

```
import 'moment/locale/fr.js';
```

and to use `--env fr`. Unfortunately, there doesn't seem to be a way to specify two different environments. And I wouldn't like to create a dev-fr environment, a prod-fr environment, a dev-en environment, a prod-en environment, etc.

Another use-case for that would be to create separate bundles for browsers, containing the polyfills that are needed for different browsers, and thus be able to generate a production bundle, for the French locale, and the chrome browser. I'm sure other use-cases could exist.

So I think it would be nice if angular-cli allowed specifying several environments. This is BTW a feature that exists in other build tools (like Maven profiles, or gradle properties, for example)

What do you think? Is there another nice way to achieve that?

Inizia dalla gestione da parte di angular-cli dell’opzione --env e dal punto di ingresso della configurazione della build. Riproduci i casi d’uso di locale e profili del browser descritti nell’issue, quindi definisci come devono combinarsi più ambienti senza richiedere che ogni coppia di ambienti venga dichiarata separatamente.

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