Feature request: nested SQL query

While the `append` method is a nice way to extend the query, sometimes I want something different. For example,

```js
const book = 'harry potter'
const author = 'J. K. Rowling'
const query = SQL`SELECT author FROM books WHERE category = (${SQL`SELECT category FROM books WHERE name = ${book}`}) AND author = ${author}`
```

Is something like this possible?