Nullable Type support

[Fatih Boy]

Hi,
    One could add nullable type support to the library by changing OptionResult.CheckType and OptionDefinition.ConvertValue functions as as shown below.

OptionResult

private void CheckType(object value) {
   if (value == null &&
    !(_defintion.ValueType.IsGenericType && _defintion.ValueType.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) &&
    typeof(ValueType).IsAssignableFrom(_defintion.ValueType)) {
    throw new InvalidValueException("Null is not supported for this type");
   }

OptionDefinition

public object ConvertValue(string value) {
   try {
    if (_valueType.IsGenericType && _valueType.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) {
     if (value == null) {
      return null;
     }

     Type underlyingType = new NullableConverter(_valueType).UnderlyingType;

     if (underlyingType.IsEnum) {
      return Enum.Parse(underlyingType, value);
     }

     return Convert.ChangeType(value,new NullableConverter(_valueType).UnderlyingType);
    }
   
    return Convert.ChangeType(value,_valueType);
   }
   catch (Exception ex) {
    throw new InvalidValueException("Invalue value: " + value,ex);
   }
  }

Regards,
Fatih Boy