support environment-specific overrides

Resolves issue #112 that was previously closed.

The idea is to declare multiple `main` overrides, in order to support different environments, such as `production`, `development`, `test`, etc.

Use case: I want to use `jquery.min.js` for my production environment, and `jquery.js` during development for easier debugging. I can now override main files in my `bower.json`:

```json
{
  "overrides": {
    "jquery": {
      "main": {
        "development": "jquery.js",
        "production": "jquery.min.js",
      }
    }
  }
}
```

I understand you didn’t want to modify wiredep’s usage of `overrides`, but this is a trivial change to the codebase and a huge advantage for developers, so I hope you’ll reconsider your decision.
This syntax is used by other libs, such as [`main-bower-files`](/ck86/main-bower-files).