Write new index.js for the package

src/index.js

@@ -1,8 +1,10 @@
 import initializeShareon from "./shareon";
 import "./style.css";
 
-window.onload = () => {
+// TODO: update README
+const s = document.currentScript;
+if (s && s.hasAttribute("init")) {
   initializeShareon();
-};
+}
 
 export default initializeShareon;