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/__tests__/index.spec.ts

@@ -14,7 +14,7 @@ describe('compiler + runtime integration', () => {
         }
       }
     }
-    createApp().mount(App, container)
+    createApp(App).mount(container)
     expect(container.innerHTML).toBe(`0`)
   })
 
@@ -33,7 +33,7 @@ describe('compiler + runtime integration', () => {
         }
       }
     }
-    createApp().mount(App, container)
+    createApp(App).mount(container)
     expect(container.innerHTML).toBe(`0`)
   })
 
@@ -51,7 +51,7 @@ describe('compiler + runtime integration', () => {
         }
       }
     }
-    createApp().mount(App, container)
+    createApp(App).mount(container)
     expect(container.innerHTML).toBe(`0`)
   })
 
@@ -60,7 +60,7 @@ describe('compiler + runtime integration', () => {
     const App = {
       template: `<div v-if>`
     }
-    createApp().mount(App, container)
+    createApp(App).mount(container)
     expect(
       `Template compilation error: Element is missing end tag`
     ).toHaveBeenWarned()
@@ -78,26 +78,24 @@ describe('compiler + runtime integration', () => {
   })
 
   it('should support custom element', () => {
-    const app = createApp()
-    const container = document.createElement('div')
-    const App = {
+    const app = createApp({
       template: '<custom></custom>'
-    }
+    })
+    const container = document.createElement('div')
     app.config.isCustomElement = tag => tag === 'custom'
-    app.mount(App, container)
+    app.mount(container)
     expect(container.innerHTML).toBe('<custom></custom>')
   })
 
   it('should support using element innerHTML as template', () => {
-    const app = createApp()
-    const container = document.createElement('div')
-    container.innerHTML = '{{msg}}'
-    const App = {
+    const app = createApp({
       data: {
         msg: 'hello'
       }
-    }
-    app.mount(App, container)
+    })
+    const container = document.createElement('div')
+    container.innerHTML = '{{msg}}'
+    app.mount(container)
     expect(container.innerHTML).toBe('hello')
   })
 })