Allow pass-in of legacyFilesToAppend via options

According to http://iamstef.net/ember-cli/#managing-dependencies, this should be possible:

```
1 var app = new EmberApp({
2   ...
3   legacyFilesToAppend: [
4     // ... existing legacy files to append
5     "moment.js" // library you wish to append
6   ],
7   //...
8 });
```

However, it looks broken on current master