create-react-app 创建react应用环境变量，env配置

参考：https://facebook.github.io/create-react-app/docs/adding-custom-environment-variables

What other .env files can be used?

Note: this feature is available with react-scripts@1.0.0 and higher.

• .env: Default.
• .env.local: Local overrides. This file is loaded for all environments except test.
• .env.development, .env.test, .env.production: Environment-specific settings.
• .env.development.local, .env.test.local, .env.production.local: Local overrides of environment-specific settings.

Files on the left have more priority than files on the right:

• npm start: .env.development.local, .env.development, .env.local, .env
• npm run build: .env.production.local, .env.production, .env.local, .env
• npm test: .env.test.local, .env.test, .env (note .env.local is missing)