Implement `echo' task action

spec/producer/core/actions/echo_spec.rb

@@ -0,0 +1,21 @@
+require 'spec_helper'
+
+module Producer::Core
+  describe Actions::Echo do
+    let(:env)       { Env.new }
+    let(:text)      { 'hello' }
+    subject(:echo)  { Actions::Echo.new(env, text) }
+
+    describe '#apply' do
+      before do
+        env.output = StringIO.new
+      end
+
+      it 'outputs the string given as argument through env.output' do
+        expect(env).to receive(:output).with(text)
+        echo.apply
+      end
+    end
+  end
+end
+