Whitelist npm package files

[MatthiasKunnen]

The npm package currently contains the following files:

├── app
│   ├── cmd.js
│   ├── index.js
│   ├── overwrite.js
│   └── visit.js
├── .editorconfig
├── .eslintrc
├── index.js
├── LICENSE
├── package.json
├── README.md
├── tests
│   ├── cmd.js
│   ├── overwrite.js
│   └── visit.js
└── .travis.yml

Files such as tests/*, .editorconfig, and.eslintrc do not serve a purpose for consumers of the package while increasing package size. Therefore, I've whitelisted the files that are required for the package to run.

index.js, README.md and LICENSE are included by default. Test using npm pack.