ushort and short not supported

```
class Options
{
[Option('p', "port", Required = false, HelpText = "port number")]
public int Port { get;set; }
}
```

If i change the type of Port to ushort or short I get an error when starting the program:

"ERROR(S):
Error setting value to option 'p, port': Check if Option or Value attribute values are set
properly for the given type."

Expected behavior:
1. clear error message that the type is not supported
2. support the types ushort and short