AddValidator methods should be fluent

[adamsitnik]

While working on #1964 I've realized that we allow the users to add completions in fluent way, but at the same time we don't do it for the validators.

Proposed change:

- void AddValidator(Action<ArgumentResult> validate)
+ Argument<T> AddValidator(Action<ArgumentResult> validate)

- void AddValidator(Action<OptionResult> validate)
+ Option<T> AddValidator(Action<OptionResult> validate)

cc @jonsequitur @jozkee