title: Sequelize add association
body: <p>I have an association set up like this: </p>
  
  <pre><code>m.User.hasMany(m.Interests, { joinTableName: 'user_interests', foreignKey: 'user_id' });
  m.Interests.hasMany(m.User, { joinTableName: 'user_interests', foreignKey: 'interest_id' });
  </code></pre>
  
  <p>Sequelize is awesome in that I can just do user.getInterests. But how can I add a new association?</p>