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/runtime-core/src/index.ts

@@ -99,7 +99,13 @@ export { registerRuntimeCompiler } from './component'
 
 // Types -----------------------------------------------------------------------
 
-export { App, AppConfig, AppContext, Plugin } from './apiCreateApp'
+export {
+  App,
+  AppConfig,
+  AppContext,
+  Plugin,
+  CreateAppFunction
+} from './apiCreateApp'
 export { VNode, VNodeTypes, VNodeProps } from './vnode'
 export {
   Component,