Use grunt's files-abstraction

I would like use [grunt files features](http://gruntjs.com/configuring-tasks#files) to optimize different modules with one require config. That would let us write things like this:

```js
requirejs: {
  target: {

    options: {
      baseUrl: 'test/fixtures',
      almond: true
    },
    
    files: {
      'tmp/requirejs-project1.js': ['test/fixtures/project1.js'],
      'tmp/requirejs-project2.js': ['test/fixtures/project2.js'],
    },
  }
}
```

I did some work in that direction and I propose it in that pull request to discuss about this feature.. :smile: