[NEW FEATURE] Support components as selectors

Thanks for a great project. We are currently trying to reference other components in the CSS, i.e.,

```js

const Inner = styled.div`
`

const Container = styled.div`
  ${Inner} {
    background-color: red;
  }
`
```

but it seems that it'll stringify the component rather than resolve a class-name. I also found no examples of this in the project. Is this not supported?
