Add options to add virtual fields to table and custom associations

[chiendv]

Hi team.

I had made and updated in my forked repository to add new options to support to:

• Add virtual fields for any generated models
• Add custom associations to any generated models ( It useful because sometimes we not allowed to update the database directly)
• Allow to overwrite models "additional" for specified tables

new options look like this:

const auto = new SequelizeAuto('database', 'user', 'pass', {
   ///...
    additional: {
        timestamps: false,
        paranoid: true
        // ...options added to each model
    },
    additionalTables: {
      customer {
        options: {
          timestamps: false,
          paranoid: false
        },
        virtualFields: {
          "full_name": {
            get: "return `${this.first_name} ${this.last_name}`" //value is plain text add to model get/set function
          }
        }
      }
    },
    additionalRelations:[
      `Customer.belongsToMany(Product, { as: 'customerProducts', through: OrderItem, foreignKey: "order_id", otherKey: "product_id" });`
    ],
    //...
})

You can test here https://www.npmjs.com/package/@chiendv/sequelize-auto/v/0.9.0-beta.1

You it is good to merge to this main repository and will create a PR.

Thank you!