Allow vendor files to be easily configurable.

This allows alternate paths to be specified, or completely removed.

For example to exclude jQuery from the generated `vendor.js` file:

```javascript
var app = new EmberApp({
  vendorFiles: {
    'jquery.js': null
  }
});
```

This is a precursor to the new configuration syntax that will could
something like `app.config('vendorFiles', 'ember.js', <options here>)`.