Support async loading

Fixes widget not initializing if loaded async or after the `DOMContentLoaded` event fires. That event fires when `readystate` changes from initial state `loading` to `interactive`, so we should only use the listener if the current state is `loading`.

Author: erquhart

src/index.js

@@ -6,7 +6,10 @@ if (typeof exports !== undefined) {
 if (typeof window !== undefined) {
   window.netlifyIdentity = netlifyIdentity;
 }
-
-document.addEventListener("DOMContentLoaded", () => {
+if (document.readyState === "loading") {
+  document.addEventListener("DOMContentLoaded", () => {
+    netlifyIdentity.init();
+  });
+} else {
   netlifyIdentity.init();
-});
+}