Add peek method in Router to display current active screen

scoop/src/main/java/com/lyft/scoop/Router.java

@@ -119,6 +119,10 @@ public boolean hasActiveScreen() {
         return !backStack.isEmpty();
     }
 
+    protected Screen peek() {
+        return backStack.peek();
+    }
+
     private boolean tryHandleEmptyBackstack(final Screen screen) {
         if (backStack.isEmpty()) {
             backStack.push(screen);