add client-side preloading logic, move router into runtime module

runtime/router.js

@@ -0,0 +1,25 @@
+const router = {
+	init(callback) {
+		console.log('initing');
+		window.addEventListener('click', event => {
+			let a = event.target;
+			while (a && a.nodeName !== 'A') a = a.parentNode;
+			if (!a) return;
+
+			if (callback(new URL(a.href))) {
+				event.preventDefault();
+				history.pushState({}, '', a.href);
+			}
+		});
+
+		window.addEventListener('popstate', event => {
+			callback(window.location);
+		});
+
+		callback(window.location);
+	}
+};
+
+window.router = router;
+
+export default router;