BREAKING CHANGE: `<portal>` has been renamed to `<teleport>`.
`target` prop is also renmaed to `to`, so the new usage will be:
```html
<Teleport to="#modal-layer" :disabled="isMobile">
<div class="modal">
hello
</div>
</Teleport>
```
The primary reason for the renaming is to avoid potential naming
conflict with [native portals](https://wicg.github.io/portals/).
BREAKING CHANGE: replae `watch(fn, options?)` with `watchEffect`
The `watch(fn, options?)` signature has been replaced by the new
`watchEffect` API, which has the same usage and behavior. `watch`
now only supports the `watch(source, cb, options?)` signautre.
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()
```
