Support inner interface

```
 import org.p2s.Settings;
 import java.util.Optional;

 @Settings
 public interface SomeSettings {

     Optional<Integer> timeout();
     String name();
     String theSurname();
     InnerSettings innerSettings();

     interface InnerSettings {
          String other();
      }
 }
```

It produces an `cannot find symbol` compilation error.
