Add the ability to inject Views into Markdown articles

Consider a markdown article like this:

```md
Lots of text

Lots more text.
```

The `div` portion of the Markdown would ideally be injectable in some fashion using a more canonical Slipstream View definition:

```swift
var body: some View {
Div {
...
}
}
```

This would require some kind of special markup that the Markdown renderer would need to handle.