Support user-defined classes

[brillout]

Allow the user to extend json-serializer with user-defined classes with custom (de-)serialization.

AFAICT it should be fairly easy to implement. PR welcome.

Currently json-serializer behaves like JSON.stringify()/JSON.parse() regarding user-defined classes:

class MyClass {
  prop = 42
}

const obj = new MyClass()
console.log(obj) // MyClass { prop: 42 }
console.log(obj instanceof MyClass) // true

const copy = JSON.parse(JSON.stringify(obj))
console.log(copy) // { prop: 42 }
console.log(copy instanceof MyClass) // false