title: Sequelize: Include model of through.attribute
body: <p>Is it possible to include a model for an attribute of a many-to-many relation via <code>through</code>?</p>
  
  <p>Let's say I'm having the following Models:</p>
  
  <pre><code>User = sequelize.define('user', {/* attributes */});
  Question = sequelize.define('question', {/* attributes */});
  Answer = sequelize.define('answer', {/* attributes */});
  
  /* Where */
  Question.hasMany(Answer);
  Answer.belongsTo(Question);
  </code></pre>
  
  <p>I'd like to save for every user which answer he gave to a question. So I added the following relation:</p>
  
  <pre><code>UserQuestionAnswerRel = sequelize.define('usersAnwers', {
    user: {/**/},
    question: {/**/},
    answer: {/**/}
  });
  
  User.hasMany(Question, { through: UserQuestionAnswerRel });
  </code></pre>
  
  <p>Now I'd like to query a user receiving all questions he answered <em>and the answer he gave</em>. But how does that work? Unfortunately this doesn't work:</p>
  
  <pre><code>User.findAll({
    include: {
      model: Question,
      through: { include: model.Answer }
    }
  }).then /* */
  </code></pre>
  
  <p><strong>Please note:</strong> All this Q&amp;A stuff is just an example. It won't be possible for me to not use a <code>through</code> relation.</p>