Allow more customization to homepage option

Created by: lucasavila00

The current setup doesn't allow to use CRA and a CDN if index.html isn't in the same url as the other assets.

Setting the homepage on package.json to "https://somethig.com" will pass "/" as publicPath to webpack. https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/config/webpack.config.prod.js#L38 That's fine.

But I'd like to serve all assets at a CDN (Firebase) and render "index.html" dynamically elsewhere (AWS Lambda) to add custom meta tags per route, as suggested here: https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md#generating-dynamic-meta-tags-on-the-server. But then I have to set my publicPath to use Firebase's link.

At webpack's docs they say they support it: https://github.com/webpack/docs/wiki/configuration

And a more complicated example of using a CDN and hashes for assets.

config.js

output: {
    path: "/home/proj/cdn/assets/[hash]",
    publicPath: "http://cdn.example.com/assets/[hash]/"
}

I wouldn't like to eject just for this, so, what do you guys think about letting me skip this logic https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/config/webpack.config.prod.js#L44 and set the publicPath I want?

Perhaps escaping the homepage field with a special char like "@https://foo.firebaseapp.com/" will disable https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/config/webpack.config.prod.js#L44

Proposed POC for this "fix" I came up with: https://github.com/facebookincubator/create-react-app/pull/1280

Thanks for the great work you've been doing on this project 👍