Upgrade `babel-loader` and remove the cache directory configuration
that was added in #620. `babel-loader` now uses the
`./node_modules/.cache/babel-loader` directory by default, so the
custom config is no longer needed.

* Create asset filenames mapping on the build output

I use danethurber/webpack-manifest-plugin on Webpack production
configuration to create a file named `asset-manifest.json` which contain a
mapping of all asset filenames to their corresponding output file.

`asset-manifest.json` will be located at the root of output folder.

This'll resolve #600

* Add an explanation for ManifestPlugin

* Make webpack-manifest-plugin's version exact

* Define process.env as object

* Fix define process.env

* fix NODE_ENV check

* Fix style nitpick

* Typo fix in webpack.config.dev.js comments

* Typo fix in webpack.config.prod.js comments

* Add syntax error overlay in development

* Support HMR being disabled

* Tweak CSS

* Extract some utilities into a separate package

* Add utils dir to `files` in package.json

* Do not create an empty `utils` dir on eject

Fixes #410, #674

* Move ESLint configuration to a separate package

* Remove the ESLint configuration, moved to eslint-config-react-app

* Update ESLint instructions

* Pin the package versions in eslint-config-react-app

* Add a README for eslint-config-react-app

* Update README.md

* Update README.md

* Update README.md

* Update README.md

* Update package.json

* Update package.json

* Update production eslint-loader config

* Add the ESLint config to devDependencies of the repo