“Unsafe” HTML rendering

[mbostock]

Related to #895 we could have an html.unsafe function that makes it easier to render HTML unsafely. It could work like this:

```js
function htmlUnsafe(string) {
  const template = document.createElement("template");
  template.innerHTML = string;
  return template.content.cloneNode(true);
}
```

This is ${htmlUnsafe(`<b>unsafe</b>`)}.