Constrain allowed values

[mookid8000]

Similar to how [Example("whatever")] can be used to show example values, an [AllowedValue("someValue")] attribute should be used to specify which values are allowed.

Could be simple like the one above - could also be more advanced as in:

[Parameter("n")]
[AllowedValue("a prime: 2, 3, 5, 7, ...", validator: typeof(PrimeChecker))]
public int N { get; set; }

where PrimeChecker could then be an IValidator:

public class PrimeChecker : IValidator
{
    public void Validate(string value, Arguments accessToTheOtherArguments)
    {
        if (!ValueIsPrime(int.Parse(value))) {
            throw new GoCommandoException($"Value '{value}' is not prime!");
        }
    }
}

Not sure yet what would work best. Could start out with the simple one.