title: Add and new column/attribute to sequelize model
body: <p>Is there a way to add a new column to a model that has already been defined?</p>
  
  <p>Something like:</p>
  
  <pre><code>var Agency = sequelize.define("agency", {
    agency_id: {
      type: DataTypes.STRING(255),
      primaryKey: true
    },
    agency_name: DataTypes.STRING(255),
    agency_url: DataTypes.STRING(255)
  });
  
  Agency.addColumn({ agency_lang: DataTypes.STRING(2) });
  </code></pre>