Make adding custom validation easier

[Bartłomiej Świerad]

I want to be able to create a custom validation logic easily.

Proposed interfaces:

validations.add(\.email) { email -> [ValidationError] in
  guard email == "[e-mail]" else { return [.emailIsNotMe] }
}

validations.add { model -> [ValidationError] in
  var errors: [ValidationError] = []
  if model.me != "me" {
    errors.append(.meIsNotMe)
  }
  if model.you != "you" {
    errors.append(.youIsNotYou)
  }
  return errors
}