Added exclude node_modules to webpack.config's babel-loader.

Created by: rouzazari

The webpack update is great. Thank you for it. I want to make one recommendation, but please let me know if I'm misunderstanding, as I'm still learning:

Starting from commit https://github.com/verekia/js-stack-from-scratch/commit/300317b4f821ec9455e4ceebd26b23458b89d817 you introduced webpack.config.babel.js to manage the bundling and babel-loader. I believe it's common practice to either exclude: /node_modules/ or include specific files or folders to avoid having babel-loader go through the whole node_modules folder. I believe this affects chapters 7-12.

I made the change below and noticed that it decreased the run time of gulp task main from 15 seconds to 2.5 seconds when excluding /node_modules/ in chapter 11. The resulting client-bundle.js is slightly different but seems to work just the same.

The bolded line below shows the change to 6 files (webpack.config.babel.js in chapter 7-12):

loaders: [
{
  test: /\.jsx?$/,
  loader: "babel-loader",
  exclude: [/node_modules/],
}