* 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

* Update react-scripts dependencies to use jest 16

* Remove testRegex from Jest config (now default)

 - react-dev-utils@0.2.1
 - react-scripts@0.6.1

Removes a deprecation warning:
npm WARN deprecated minimatch@0.3.0: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue

See: https://github.com/jergason/recursive-readdir/pull/39

 - babel-preset-react-app@0.2.1
 - create-react-app@0.5.0
 - eslint-config-react-app@0.2.1
 - react-dev-utils@0.1.0
 - react-scripts@0.5.0

* Extract some utilities into a separate package

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

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

* Load environment file via dotenv if .env file is present

* Document loading environment variables in .env file

* Minor doc tweaks

Fixes #410, #674

* Update eslint-plugin-react and enable new rules

New rules:
* `react/no-danger-with-children` (yannickcr/eslint-plugin-react#710)
* `react/no-find-dom-node` (yannickcr/eslint-plugin-react#678)
* `react/style-prop-object` (yannickcr/eslint-plugin-react#715)

* Remove react/no-find-dom-node for now

* 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

* Revert "Add ES5 version of `path-exists` to CLI"

This reverts commit fc3ab46d.

* Use pre node@4 compatible `path-exists`

Ref #617

Resolves an issue that can occur in certain situations that causes a build error when user has CDed into an incorrectly cased directory.

* Specify cache directory for babel loader

* Use `react-scripts` for folder name instead

tagging dev-only code (@remove-on-publish-begin/end) and pre-eject-only code (@remove-on-eject-begin/end) #257 (#257)