Support for using a readable stream

It might just be something to add in the docs but is there any possible way to use this as part of a transformation stream? If I wanted to implement sharp in the following way for example:

```js
const transform = sharp().rotate();
const smartcrop = smartcrop( result => {
transform.extract( result.topCrop )
} )
readableStream.pipe( smartcrop ).pipe( transform ).pipe( writableStream );
```

No worries if it's difficult / impossible to support. I'm trying to make my service a less memory intensive for large files.