Support for pluggable renderers

Like `inferno` or `preact`.

From the end user perspective, it would work as follows.
Configure `package.json`:

``` js
{
  "dependencies:" {
    "next": "x.y.z",
    "next-preact": "x.y.z",
    "preact": "x.y.z"
  }
}
```

Then we transform JSX to the pragma that, in this example, `next-preact` would need.
The rendering pipeline would similarly be pluggable.
