Implement nested tasks

features/task_nested_tasks.feature

@@ -0,0 +1,15 @@
+Feature: nested tasks
+
+  Background:
+    Given a recipe with:
+      """
+      task :outer_task do
+        task :inner_task do
+          echo 'OK'
+        end
+      end
+      """
+
+    Scenario: applies nested tasks
+      When I successfully execute the recipe
+      Then the output must match /\AOK/