Support for preloaded applications (cache manifest early)

We wanted to make sure that the asset manifest is loaded early in our Rails boot process, so that forking servers (like unicorn) have the manifest data cached for new workers, instead of going to disk for it.

I'm using a private api method in an initializer to accomplish this currently:

```
Webpack::Rails::Manifest.send(:manifest)
```

Could a public/supported method be added for this?