use es2015 syntax for webpack configs

[domasx2]

Hi!

I think webpack configs are nicer to manage and read if spread operator is used to extend them.

//instead of
const config = Object.create(baseConfig);
config.entry = ['another_entrypoint.js'];
config.plugins.push(new SomePlugin()};
module.exports = config;

//use this
export default {
   ...baseConfig,
   entry: ['another_entrypoint.js'],
   plugins: [...baseConfig.plugins, new SomePlugin()]
};

This is easy to add too since babel is already in the project.

This PR:

• babel-node used to run webpack, instead of node
• webpack configs rewritten using es2015/es2017 syntax

[chentsulin]

Thanks!