feat(transition): add TransitionStub to export

Author: eddyerburgh

src/index.js

@@ -1,9 +1,13 @@
 import shallow from './shallow'
 import mount from './mount'
 import createLocalVue from './create-local-vue'
+import TransitionStub from './components/TransitionStub'
+import TransitionGroupStub from './components/TransitionGroupStub'
 
 export default {
   createLocalVue,
   mount,
-  shallow
+  shallow,
+  TransitionStub,
+  TransitionGroupStub
 }