fix: add default export

Author: wxnet2013

src/index.js

@@ -1,4 +1,4 @@
-export function loading(
+export default function loading(
   config = {
     name: 'loading',
   }

test/index.spec.js

@@ -1,5 +1,5 @@
 import rxloop from '@rxloop/core';
-import { loading } from '../src/';
+import loading from '../src/';
 import { delay, mapTo } from "rxjs/operators";
 
 const app1 = rxloop();