title: Sequelize order by count association?
body: <p>I have <code>User</code> and <code>Post</code> table. 
  <code>User has Many Posts</code> relationship. I want to query User and order the query by the number of posts that the users made. How do I write this sequelize query?</p>
  
  <pre><code>Post.belongsTo(User, {foreignKey: 'userId'});
  User.hasMany(Post, {foreignKey: 'userId'});
  
  User.findAll( {
    order: 'COUNT(Post) DESC', //???
    include: [Post]
  });
  </code></pre>
  
  <p>How do I get this User query sorted by Post?</p>