Automatically insert overlay in to webpack entry property

As @shellscape mentioned in [this issue](https://github.com/webpack-contrib/webpack-hot-client/issues/19#issuecomment-367739296); [`webpack-hot-client`](https://github.com/webpack-contrib/webpack-hot-client) does automatic entry injection.

It would be nice if this package did the same, so that users wouldn't have to add the entry manually to their webpack configurations.

Start with the linked webpack-hot-client issue and its automatic entry injection behavior, then inspect how this package integrates with the webpack entry property. Done means users no longer need to add the overlay entry manually to their webpack configurations; the payload does not name a file or test to run.

Written by the indexing model from the issue text.