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/commits.html

@@ -32,7 +32,7 @@ const truncate = v => {
 
 const formatDate = v => v.replace(/T|Z/g, ' ')
 
-const App = {
+createApp({
   setup() {
     const currentBranch = ref('master')
     const commits = ref(null)
@@ -54,9 +54,7 @@ const App = {
       formatDate
     }
   }
-}
-
-createApp().mount(App, '#demo')
+}).mount('#demo')
 </script>
 
 <style>