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?

Start with the angular-cli handling for the --env option and the build configuration entry point. Reproduce the locale and browser-profile use cases described in the issue, then define how multiple environments should combine without requiring every environment pair to be declared separately.

Written by the indexing model from the issue text.