Support preload headers

[matthewp]

Since we know the bundles the user will need to render the page we could add the preload headers:

Link: </dist/bundles/my-app/index.css>; rel=preload; as=style
Link: </dist/bundles/my-app/index.js>; rel=preload; as=script

This would require adding configuration for the bundles path:

ssr({}, {
  bundlesPath: __dirname + "/dist"
});