Add ESLint extend support to eslint-loader

Created by: mrmckeb

This allows users to extend the base ESLint config, so long as it is extending the react-app config.

Config in package.json Initially, this is limited to users that use package.json to configure ESLint. We can extend this later, but it will have performance implications as we would need to:

• Find which config file the user is using.
• Read that file, and confirm that it extends the react-app config.
• Set that config to eslint-loader.

Eslint provides tooling to support this, but again, this will impact performance.

Users could work around this, by setting the following in package.json:

{
 "eslintConfig": {
   "extends": ["react-app", "./my-eslint-config.js"]
 }
}

Documentation Once this approach is approved, I'll update our ESLint documentation to outline usage.