Allow addons to be vendored outside of node modules

In package.json you can add paths for additional addons

```json
"ember-addon": {
  "paths": [
    "./lib/super-button"
  ]
}
```

The vendored library must conform to the addon spec, so it must have a
properly declared `package.json` file as well as its own loader.
Can you tweak this to `addIfAddon`?