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?

從 angular-cli 對 --env 選項的處理以及建置組態入口開始。重現 issue 中描述的 locale 和瀏覽器設定檔使用案例，然後定義多個環境應如何組合，而不要求個別宣告每個環境配對。

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