Use file-loader as fallback for files that match no other loader

[davidtheclark]

This PR to create-react-app introduced a clever method for using the file-loader as a fallback, instead of relying on a whitelist or blacklist of extensions. We should do this!

Doing this right would men removing the fileLoaderExtensions config option, so would mean a breaking change. This probably isn't worth doing, then, until it can be grouped with other breaking changes.