Implement any.of

Rx knows [`any.of`](http://rx.codesimply.com/coretype/any.html) for subschemas, and [feature request #10 on the Kwalify sourceforge](https://sourceforge.net/p/kwalify/feature-requests/10/) suggests an implementation.

This would be a great feature to have for cases where, e.g., a data set will have a *type*, and this type only allows for a certain subset of keys to be used.

````yaml
type: vehicle-factory
products:
- type: car
doors: 4
wheels: 4
- type: vanette
doors: 5
wheels: 4
- type: motorcycle
handle: steel
wheels: 2
````

In this (arguably not very clever) example, `car`s can have `doors`, so can `vanette`s. `motorcycles` cannot have `doors`, but can have `handle`s made from a specific material. Both the other vehicle types cannot have `handle`s, but all vehicles can have `wheels`, etc.

This feature would allow for much stricter schemas to be implemented and validated, and I think would be of great use to many users.