Add an "eject" command

[create-react-app](https://github.com/facebook/create-react-app) has an "eject" command, where one basically calls "create-react-app eject" (or something similar, I haven't actually used this) and all the dynamic configuration that create-react-app creates for you are written to disk, so that eventually when your project grows and you want more control over your build chain you can have it pretty easily.

I think a similar command for `electron-webpack` where it outputs to disk the generate webpack configurations for the main process and the renderer process both for development and production would be great.

Start by locating where electron-webpack generates webpack configurations for the main and renderer processes, then trace how development and production settings are selected. Define the eject command's output locations and verify that all four generated configurations are written to disk and can be used independently.

Written by the indexing model from the issue text.