How to use Input and output container in Swift

Issue #87

This is a very nifty trick from ios-oss which was built around MVVM pattern. It uses protocol to define input and output, and a container protocol to contain them. Take https://github.com/kickstarter/ios-oss/blob/1f5643f6a769995ccd1bb3826699745e64597ab7/Library/ViewModels/LoginViewModel.swift for example

public protocol LoginViewModelInputs {

}

public protocol LoginViewModelOutputs {

}

public protocol LoginViewModelType {
  var inputs: LoginViewModelInputs { get }
  var outputs: LoginViewModelOutputs { get }
}

public final class LoginViewModel: LoginViewModelType, LoginViewModelInputs, LoginViewModelOutputs {
  public var inputs: LoginViewModelInputs { return self }
  public var outputs: LoginViewModelOutputs { return self }
}

Look how LoginViewModel conforms to 3 protocols. And when you access its input or output properties, you are constrained to only LoginViewModelInputs and LoginViewModelOutputs