Add UMD build

package.json

@@ -114,7 +114,9 @@
     "yargs": "^15.0.2"
   },
   "scripts": {
-    "build": "cross-env NODE_ENV=production npm run clean && rollup -c",
+    "build": "npm run clean && npm run build:base && npm run build:umd",
+    "build:base": "cross-env NODE_ENV=production rollup -c",
+    "build:umd": "cross-env NODE_ENV=production rollup -c rollup.config.umd.js",
     "watch": "rollup -cw",
     "clean": "rimraf es && rimraf lib",
     "lint": "eslint --ext .js --ext .jsx src test",

rollup.config.umd.js

@@ -0,0 +1,22 @@
+import resolve from 'rollup-plugin-node-resolve';
+import packageJSON from './package.json';
+import baseConfig from './rollup.config';
+
+export default {
+  input: 'src/index.js',
+  output: [
+    {
+      file: 'lib/index.umd.js',
+      format: 'umd',
+      name: 'Floater',
+      intro: 'var global = typeof self !== undefined ? self : this;',
+      globals: {
+        react: 'React',
+        'prop-types': 'PropTypes',
+        'react-dom': 'ReactDOM',
+      },
+    },
+  ],
+  external: [...Object.keys(packageJSON.peerDependencies)],
+  plugins: [...baseConfig.plugins, resolve()],
+};