refactor: adjust createApp related API signatures

BREAKING CHANGE: `createApp` API has been adjusted.

  - `createApp()` now accepts the root component, and optionally a props
  object to pass to the root component.
  - `app.mount()` now accepts a single argument (the root container)
  - `app.unmount()` no longer requires arguments.

  New behavior looks like the following:

  ``` js
  const app = createApp(RootComponent)
  app.mount('#app')
  app.unmount()
  ```

packages/vue/examples/composition/todomvc.html

@@ -90,7 +90,7 @@ function pluralize (n) {
   return n === 1 ? 'item' : 'items'
 }
 
-const App = {
+createApp({
   setup () {
     const state = reactive({
       todos: todoStorage.fetch(),
@@ -202,7 +202,5 @@ const App = {
       }
     }
   }
-}
-
-createApp().mount(App, '#app')
+}).mount('#app')
 </script>