server.js: Add --config flag for different local webpack config files

[kazarmy]

This pr adds a --config/-c flag to server.js to allow use of local webpack config files other than webpack.local-config.js.

This addition is primarily to support scripts that invoke server.js to safely override the settings in webpack.local-config.js, since modifying webpack.local-config.js directly is fraught with danger (e.g. what happens if the invoking script or server.js abruptly terminates?). Specifically, this addition is to support the request here (hopefully I've understood correctly) to have a separate Node.js script run server.js.

Another use case is to easily switch between different webpack config files that perhaps open different browsers. But honestly this is secondary to the above.

[codecov]

Codecov Report

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Additional details and impacted files

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[julienw]

Thanks for the patch!

My main concern is about the security of this: indeed require will gladly execute anything that's fed to it. But I think it's fine as long as we don't accept environment variables to provide this information.

Also it's not 100% clear what you'll do with that to support my comment #4452 (review) but I'll wait and see :-)