title: Sequelize composite unique constraint
body: <p>Defining model:</p>
  
  <pre><code>export default function(sequelize, DataTypes) {
     return sequelize.define('Item', {
         minor: DataTypes.INTEGER,
         major: DataTypes.INTEGER,
     });
  }
  </code></pre>
  
  <p>Can I define the pair minor and major as a composite UNIQUE constraint?</p>