Allow usage of array of configs for webpack

[jvdsande]

Webpack allows users to provide an array of configs instead of a single config.

This PR allows users to provide an array of config through the webpack.config configuration hook.

• In development, it gets the publicPath option for the devServer from the first config, if an array is provided
• In production, it prints the gzipped size information for each configs sequentially

As far as I can tell it doesn't break anything.

[jvdsande]

CI tests are failing, but they seem to also be failing on the next branch unmodified, at the same point, so this doesn't seem to be linked to those changes.