Add Env#output with $stdout as default

spec/producer/core/env_spec.rb

@@ -13,6 +13,10 @@ module Producer::Core
         it 'has no recipe' do
           expect(env.current_recipe).not_to be
         end
+
+        it 'assigns $stdout as the default output' do
+          expect(env.instance_eval { @output} ).to eq $stdout
+        end
       end
 
       context 'when a recipe is given as argument' do
@@ -25,6 +29,16 @@ module Producer::Core
       end
     end
 
+    describe '#output' do
+      let(:standard_output) { StringIO.new }
+
+      it 'writes the given string to the assigned IO with a record separator' do
+        env.output = standard_output
+        expect(standard_output).to receive(:puts).with('some content')
+        env.output 'some content'
+      end
+    end
+
     describe '#target' do
       let(:target) { Object.new }