[Feature]: Watch for webpack config changes in watch mode

I'm submitting a feature request

Webpack version:
2.x

Please tell us about your environment:
OSX 10.x

Current behavior:
I'm not sure if this was discussed before, but it's one of the pain point for me in webpack workflow.

When config / config's dep is updated — hot reloadable server (or watch mode) must be restarted manually. Usually this hot reloadable server is not stand-alone process, but part of the bigger task or even lives inside docker container. And in case of config related change — the only way to pick this up is to restart everything.

Expected/desired behavior:

It'd be cool if webpack could track such changes and rebuild everything from scratch w/o killing current process.

What is the motivation / use case for changing the behavior?

It'll make easier:

• work on webpack config setup
• work w/ loader's configs, eg postcss plugin's options (js vars etc)

Moved from https://github.com/webpack/webpack/issues/3153

No file or test is named; begin at webpack-cli's watch-mode entry point and inspect how webpack configuration and its dependencies are watched. Reproduce a configuration or loader/PostCSS option change in the stated webpack 2.x setup, then verify watch mode rebuilds without restarting the current process.