Feature request: support source-map-loader

Created by: stellarhoof

We develop and maintain a number of open source libraries, some of which we use in our main product. We'd like to be able to support source maps for our open source libraries under the node_modules directory. This feature request has come up before and there has been a PR for it already but it was closed (https://github.com/facebook/create-react-app/pull/2355).