fix: Set unpkg default to feather.min.js

Prevents webpack from referencing `window` in the UMD build which was causing an error in node.

package.json

@@ -3,6 +3,7 @@
   "version": "0.0.0-development",
   "description": "Simply beautiful open source icons",
   "main": "dist/feather.js",
+  "unpkg": "dist/feather.min.js",
   "files": [
     "dist"
   ],

webpack.config.js

@@ -6,6 +6,9 @@ module.exports = {
     path: path.resolve(__dirname, 'dist'),
     libraryTarget: 'umd',
     library: 'feather',
+    // Prevents webpack from referencing `window` in the UMD build
+    // Source: https://git.io/vppgU
+    globalObject: "typeof self !== 'undefined' ? self : this",
   },
   devtool: 'source-map',
   module: {