Support portals

A common use-case is to render given fragment somewhere outside of currently rendered tree. For example, add context menu somewhere in parent to maintain proper stacking context. Should support something like this:

```xml
<template>
  <p>Lorem ipsum</p>
  <e:portal target={someElem}>
    <submenu animate:in="..." animate:out="..." />
  </e:portal>
</template>
```