Add support for ThroughOptions

[RobinBuschmann]

See http://docs.sequelizejs.com/manual/tutorial/associations.html#n-m

Post.belongsToMany(Tag, {
  through: {
    model: ItemTag,
    unique: false,
    scope: {
      taggable: 'post'
    }
  },
  foreignKey: 'taggable_id',
  constraints: false
});

and https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/sequelize/index.d.ts#L1374:

interface ThroughOptions {

        /**
         * The model used to join both sides of the N:M association.
         */
        model: Model<any, any>;

        /**
         * A key/value set that will be used for association create and find defaults on the through model.
         * (Remember to add the attributes to the through model)
         */
        scope?: AssociationScope;

        /**
         * If true a unique key will be generated from the foreign keys used (might want to turn this off and create
         * specific unique keys when using scopes)
         *
         * Defaults to true
         */
        unique?: boolean;

    }