Support nullable members should be optional

TypeScript has something called "Optional Members":

```javascript
export interface Poco {
name: string;
birthdate? : Date; // Optional
}
```

If my poco has a nullable member
```csharp
public class Poco {
   public string Name {get;set;}
   public DateTime? Birthdate {get;set}
}
```

The current implementation would not make the nullable member optional.


