Implement target recipe keyword feature

spec/producer/core/env_spec.rb

@@ -5,10 +5,25 @@ module Producer::Core
     let(:recipe)  { Recipe.new(proc { nil }) }
     subject(:env) { Env.new(recipe) }
 
+    describe '#initialize' do
+      it 'has no target' do
+        expect(env.target).not_to be
+      end
+    end
+
     describe '#current_recipe' do
       it 'returns the assigned current recipe' do
         expect(env.current_recipe).to eq recipe
       end
     end
+
+    describe '#target' do
+      let(:target) { Object.new }
+
+      it 'returns the defined target' do
+        env.target = target
+        expect(env.target).to eq target
+      end
+    end
   end
 end