Implement basic task actions API:

* Add Action base class;
* Implement Task::DSL.define_action(keyword, klass);
* Add Task::DSL#actions accessor;
* Apply task DSL actions when Task instances are evaluated.

spec/producer/core/task/dsl_spec.rb

@@ -2,9 +2,23 @@ require 'spec_helper'
 
 module Producer::Core
   describe Task::DSL do
+    let(:block)   { proc { } }
     let(:env)     { double('env') }
     subject(:dsl) { Task::DSL.new &block }
 
+    describe '.define_action' do
+      it 'defines a new action keyword' do
+        Task::DSL.define_action(:some_action, Object)
+        expect(dsl).to respond_to :some_action
+      end
+    end
+
+    describe '#initialize' do
+      it 'has no action' do
+        expect(dsl.actions).to eq []
+      end
+    end
+
     describe '#evaluate' do
       let(:block) { proc { throw :task_code } }
 
@@ -13,6 +27,24 @@ module Producer::Core
           .to throw_symbol :task_code
       end
 
+      context 'when a defined keyword action is called' do
+        let(:some_action_class) { Class.new(Action) }
+        let(:block)             { proc { some_action } }
+
+        before do
+          Task::DSL.define_action(:some_action, some_action_class)
+          dsl.evaluate(env)
+        end
+
+        it 'registers the action' do
+          expect(dsl.actions.first).to be_an Action
+        end
+
+        it 'provides the env to the registered action' do
+          expect(dsl.actions.first.env).to eq env
+        end
+      end
+
       context 'when given block is invalid' do
         it 'raises a TaskEvaluationError on NameError' do
           dsl = Task::DSL.new { invalid_action }