Don't assume the project is hosted at the root

Created by: gaearon

We currently have an assumption in publicPath in webpack.config.prod.js that the bundle is served from the root, but it may not be (e.g. with GH Pages, it’ll be at you.github.io/projectname/).

I don’t want to add configuration for this but it would be great if we could parse homepage from package.json and use it to determine the publicPath. We can then document it or add a prompt (or a notice) as part of the creation flow.