Implement basic condition DSL framework for tests:

Instead of interrupting task evaluation when condition is not met,
allow the whole task to be evaluated (including condition and
evaluation) so that the interpreter will get all tasks actions (whether
condition is met or not) and be able to query the condition.

* Modify Interpreter#process_task: test if task condition is met before
  applying the actions;
* Implement condition handling in Task and Task::DSL;
* Implement Condition and Condition::DSL (useless as they are, but
  needed to implement later test keywords as part of the condition DSL.

spec/producer/core/task_spec.rb

@@ -4,7 +4,8 @@ module Producer::Core
   describe Task do
     let(:name)      { :some_task }
     let(:action)    { double('action') }
-    subject(:task)  { Task.new(name, [action]) }
+    let(:condition) { double('condition') }
+    subject(:task)  { Task.new(name, [action], condition) }
 
     describe '.evaluate' do
       let(:env)   { double('env') }
@@ -34,12 +35,20 @@ module Producer::Core
         expect(task.instance_eval { @actions }).to eq [action]
       end
 
+      it 'assigns the condition' do
+        expect(task.instance_eval { @condition }).to eq condition
+      end
+
       context 'when only the name is given as argument' do
         subject(:task)  { Task.new(name) }
 
         it 'assigns no action' do
           expect(task.actions).to be_empty
         end
+
+        it 'assigns a truthy condition' do
+          expect(task.condition).to be_true
+        end
       end
     end
 
@@ -54,5 +63,29 @@ module Producer::Core
         expect(task.actions).to eq [action]
       end
     end
+
+    describe '#condition' do
+      it 'returns the assigned condition' do
+        expect(task.condition).to be condition
+      end
+    end
+
+    describe '#condition_met?' do
+      context 'when condition is truthy' do
+        let(:condition) { Condition.new(true) }
+
+        it 'returns true' do
+          expect(task.condition_met?).to be true
+        end
+      end
+
+      context 'when condition is falsy' do
+        let(:condition) { Condition.new(false) }
+
+        it 'returns false' do
+          expect(task.condition_met?).to be false
+        end
+      end
+    end
   end
 end