Support inline expressions for attributes (and in other places)

[mbostock]

Currently inline expression ${…} are only supported for node content. So you can’t do this:

```js
const link = "https://example.com";
```
This is a <a href=${link}>link</a>.

You can workaround it using Hypertext Literal like so:

```js
const link = "https://example.com";
```
This is a ${htl.html`<a href=${link}>link</a>`}.

I’m not immediately sure how we would support this. It might involve writing our own Markdown parser? Or at least pre-processing the content to remove the live code expressions before passing it to markdown-it? Presumably we could repurpose parts of Hypertext Literal to parse the HTML.

[mbostock]

This is one of the trickiest outstanding questions in the CLI. If we do this, it probably requires radical changes to how we parse Markdown and generate HTML & JavaScript. We should decide whether we want to do that before Early Access.