Added subtitle option to screenParams (#567)

android/app/src/main/java/com/reactnativenavigation/params/BaseScreenParams.java

 public class BaseScreenParams {
     public String screenId;
     public String title;
+    public String subtitle;
     public NavigationParams navigationParams;
     public List<TitleBarButtonParams> rightButtons;
     public TitleBarLeftButtonParams leftButton;

android/app/src/main/java/com/reactnativenavigation/params/parsers/ScreenParamsParser.java

 
 public class ScreenParamsParser extends Parser {
     private static final String KEY_TITLE = "title";
+    private static final String KEY_SUBTITLE = "subtitle";
     private static final String KEY_SCREEN_ID = "screenId";
     private static final String KEY_NAVIGATION_PARAMS = "navigationParams";
     private static final String STYLE_PARAMS = "styleParams";
         result.styleParams = new StyleParamsParser(params.getBundle(STYLE_PARAMS)).parse();
 
         result.title = params.getString(KEY_TITLE);
+        result.subtitle = params.getString(KEY_SUBTITLE);
         result.rightButtons = ButtonParser.parseRightButton(params);
         result.overrideBackPressInJs = params.getBoolean(OVERRIDE_BACK_PRESS, false);
         result.leftButton = ButtonParser.parseLeftButton(params);

android/app/src/main/java/com/reactnativenavigation/screens/Screen.java

     private void createTitleBar() {
         addTitleBarButtons();
         topBar.setTitle(screenParams.title);
+        topBar.setSubtitle(screenParams.subtitle);
     }
 
     private void addTitleBarButtons() {