Option Default type conversion

Hi,
I'm wondering why the `Default` property of `OptionAttribute` is of type `object`. This makes it rather hard to have defaults for reference types as it has to be compile-time constant. Wouldn't it be better to have the `Default` property of type `string` and use the same conversion that is in place when the actual command line argument is parsed?

For example I'd like to have option of type `CultureInfo` with default value `en-US`, now it is not possible to have this default specified through attribute (I have to assign in in constructor of the option class)

Another quite elegant solution is adding `DefaultProperty` property to `OptionAttribute` that will specify name of property returning actual default value, this has been mentioned in issue #530 and similar to #191