Allow bundles to be analyzed with Webpack-specific tools

Created by: joshwcomeau

Associated issue: #1858 (closed)

To analyze Webpack bundles, a "stats" JSON is required.

This PR allows that file to be created and saved to the build directory, so that users can use it with Webpack-specific insight tools like webpack-bundle-analyzer without ejecting their application.

Updated the README to include details for how to do this.

Test plan

1. check that the stats flag works

I ran yarn build --stats in the root directory of the project. Once it completed, I verified that build/bundle-stats.json existed. I dragged it into Webpack Visualizer to check that the file is correct:

2. check that it works in a created project

I ran yarn create-react-app test to generate a new app.

After cd-ing into the directory and installing its dependencies, I followed the steps I added to the the README for using webpack-bundle-analyzer. Specifically I ran:

$ npm install --save webpack-bundle-analyzer

Then, I opened the project's package.json and added a script:

"analyze": "npm run build -- --stats && webpack-bundle-analyzer build/bundle-stats.json",

Finally, I ran the new script:

$ npm run analyze

The project built, and a new browser window automatically opened, with the bundle stats: