Allow merging case classes

I hope I didn't miss a way of doing this already, but it would be great if there was a possibility to merge case classes.

Example:
```
case class Person(name: String)
case class Address(street: String)
case class PersonWithAddress(name: String, street: String)

Person("John von Neumann")
  .into[PersonWithAddress]
  .transform(Fields.from(Address("1 Einstein Drive")))
``` 