Allow setting a validator class for an option.

[jarrodhroberson]

I would like to register a class to apply to the value of an option.

Implementations of the following interface would allow extremely flexible pre-validation without having to add this noise to your code and would allow better reuse of common validations.

public interface OptionValidator<T>
{
    public boolean isValid(@Nullable final T value);
    @Nonnull
    public String getErrorMessage();
}

If you could point me to where this should be added in I can write it and submit a pull request, but the indirection is obfuscating where this would go.