Export as UMD instead of just commonjs

package.json

@@ -4,7 +4,7 @@
   "description": "A React compatibility layer for Preact",
   "main": "preact-compat.js",
   "scripts": {
-    "build": "babel -f $npm_package_main -s -o $npm_package_main < src/index.js",
+    "build": "babel -f $npm_package_main -m umd -s -o $npm_package_main < src/index.js",
     "test": "eslint {src,test} && mocha --compilers js:babel/register test/**/*.js",
     "prepublish": "npm run build",
     "release": "npm run build && git commit -am $npm_package_version && git tag $npm_package_version && git push && git push --tags && npm publish"