Add OneOf() assert

[nunofgs]

It would be cool to have an assert that validates if at least one of its child nodes is present. For example:

validate({
  phone: new Assert().OneOf({
    main: [new Assert().IsString(), new Assert().Length({ max: 50 })],
    mobile: [new Assert().IsString(), new Assert().Length({ max: 48 })]
  })
});

This would make sure that at least a main or an object nodes were given.

[rplopes]

Is this still relevant? Does it solve any particular problem we're facing?