Value Combiner registration
## 🚀 Feature Requests

There is no clear way to register custom ValueCombiners. I think that this should be added in the Setup class so that we can override it. Either this or improve the documentation by saying that the ValueCombiners are not registered with reflection by default like the ValueConverters and a way to do so.

### Platforms affected (mark all that apply)
- [x] :iphone: iOS
- [x] :robot: Android
- [x] :checkered_flag: WPF
- [x] :earth_americas: UWP
- [x] :apple: MacOS
- [x] :tv: tvOS
- [x] :monkey: Xamarin.Forms
- [x] :🌍: Blazor
