Add casting support

I've integrated with a very bad API. Their JSON didn't have any types, it was all strings. It would be very usefull to have something like this:

```json
{
  "ugly": "true",
}
```

```dart
@JsonSerializable()
class Example {
  Example({this.pretty});

  @JsonKey(
    name: 'ugly',
    fromJson: (v) => v == 'true',
    toJson: (v) => v.toString(),
  )
  final bool pretty;
}
```