Add topBar.topMargin option

docs/docs/styling.md

     borderColor: 'red',
     borderHeight: 1.3,
     elevation: 1.5, // TopBar elevation in dp
+    topMargin: 24, // top margin in dp
     title: {
       height: 70 // TitleBar height in dp
     }

lib/android/app/src/main/java/com/reactnativenavigation/parse/TopBarOptions.java

         options.borderColor = ColorParser.parse(json, "borderColor");
         options.borderHeight = FractionParser.parse(json, "borderHeight");
         options.elevation = FractionParser.parse(json, "elevation");
+        options.topMargin = NumberParser.parse(json, "topMargin");
         options.buttons = TopBarButtons.parse(typefaceLoader, json);
 
         options.rightButtonColor = ColorParser.parse(json, "rightButtonColor");
     public Bool drawBehind = new NullBool();
     public Number height = new NullNumber();
     public Fraction elevation = new NullFraction();
+    public Number topMargin = new NullNumber();
     public Fraction borderHeight = new NullFraction();
     public Colour borderColor = new NullColor();
 
         if (other.borderHeight.hasValue()) borderHeight = other.borderHeight;
         if (other.borderColor.hasValue()) borderColor = other.borderColor;
         if (other.elevation.hasValue()) elevation = other.elevation;
+        if (other.topMargin.hasValue()) topMargin = other.topMargin;
 
         if (other.rightButtonColor.hasValue()) rightButtonColor = other.rightButtonColor;
         if (other.leftButtonColor.hasValue()) leftButtonColor = other.leftButtonColor;
         if (!borderHeight.hasValue()) borderHeight = defaultOptions.borderHeight;
         if (!borderColor.hasValue()) borderColor = defaultOptions.borderColor;
         if (!elevation.hasValue()) elevation = defaultOptions.elevation;
+        if (!topMargin.hasValue()) topMargin = defaultOptions.topMargin;
 
         if (!rightButtonColor.hasValue()) rightButtonColor = defaultOptions.rightButtonColor;
         if (!leftButtonColor.hasValue()) leftButtonColor = defaultOptions.leftButtonColor;

lib/android/app/src/main/java/com/reactnativenavigation/presentation/StackOptionsPresenter.java

 import android.view.Gravity;
 import android.view.View;
 import android.view.ViewGroup.LayoutParams;
+import android.view.ViewGroup.MarginLayoutParams;
 
 import com.reactnativenavigation.parse.Alignment;
 import com.reactnativenavigation.parse.AnimationsOptions;
     private void applyTopBarOptions(TopBarOptions options, AnimationsOptions animationOptions, Component component, Options componentOptions) {
         topBar.setHeight(options.height.get(LayoutParams.WRAP_CONTENT));
         topBar.setElevation(options.elevation.get(DEFAULT_ELEVATION));
+        if (topBar.getLayoutParams() instanceof MarginLayoutParams) {
+            ((MarginLayoutParams) topBar.getLayoutParams()).topMargin = UiUtils.dpToPx(activity, options.topMargin.get(0));
+        }
 
         topBar.setTitleHeight(options.title.height.get(LayoutParams.WRAP_CONTENT));
         topBar.setTitle(options.title.text.get(""));
     private void mergeTopBarOptions(TopBarOptions options, AnimationsOptions animationsOptions, Component component) {
         if (options.height.hasValue()) topBar.setHeight(options.height.get());
         if (options.elevation.hasValue()) topBar.setElevation(options.elevation.get());
+        if (options.topMargin.hasValue() && topBar.getLayoutParams() instanceof MarginLayoutParams) {
+            ((MarginLayoutParams) topBar.getLayoutParams()).topMargin = UiUtils.dpToPx(activity, options.topMargin.get());
+        }
 
         if (options.title.height.hasValue()) topBar.setTitleHeight(options.title.height.get());
         if (options.title.text.hasValue()) topBar.setTitle(options.title.text.get());

lib/android/app/src/main/java/com/reactnativenavigation/utils/UiUtils.java

         final int resourceId = resources.getIdentifier("status_bar_height", "dimen", "android");
         statusBarHeight = resourceId > 0 ?
                 resources.getDimensionPixelSize(resourceId) :
-                (int) dpToPx(context, Build.VERSION.SDK_INT >= Build.VERSION_CODES.M ? STATUS_BAR_HEIGHT_M : STATUS_BAR_HEIGHT_L);
+                dpToPx(context, Build.VERSION.SDK_INT >= Build.VERSION_CODES.M ? STATUS_BAR_HEIGHT_M : STATUS_BAR_HEIGHT_L);
         return statusBarHeight;
     }
 
         final int resourceId = resources.getIdentifier("action_bar_size", "dimen", "android");
         toolBarHeight = resourceId > 0 ?
                 resources.getDimensionPixelSize(resourceId) :
-                (int) dpToPx(context, DEFAULT_TOOLBAR_HEIGHT);
+                dpToPx(context, DEFAULT_TOOLBAR_HEIGHT);
         return toolBarHeight;
     }
 
         return dp * scale + 0.5f;
     }
 
+    public static int dpToPx(Context context, int dp) {
+        float scale = context.getResources().getDisplayMetrics().density;
+        return (int) (dp * scale + 0.5f);
+    }
+
     public static float dpToSp(Context context, float dp) {
         return dpToPx(context, dp) / context.getResources().getDisplayMetrics().density;
     }