Use files array in package.json instead of .npmignore (#692)

Whitelists are better than blacklists. Tested to ensure the same files
are bundled with `npm pack`.

Author: RyanZim

.npmignore

@@ -53,6 +53,10 @@
     "standard": "^12.0.1"
   },
   "main": "./lib/index.js",
+  "files": [
+    "lib/",
+    "!lib/**/__tests__/"
+  ],
   "scripts": {
     "full-ci": "npm run lint && npm run coverage",
     "coverage": "istanbul cover -i 'lib/**' -x '**/__tests__/**' test.js",