Also fetch manifest property again (#92)

src/connect.ts

@@ -18,7 +18,7 @@ export const connect = async (button: InstallButton) => {
 
   const el = document.createElement("ewt-install-dialog");
   el.port = port;
-  el.manifestPath = button.getAttribute("manifest")!;
+  el.manifestPath = button.manifest || button.getAttribute("manifest")!;
   el.addEventListener(
     "closed",
     () => {