Add support to define before/after graphql-sequelize hooks

[jjcollinge]

Add support to define before/after graphql-sequelize before and after hooks on a model.
With this change, if you have added a before or after find hook then it will be invoked.

This allows you to define the hooks on the model graphql object like so:

 model.graphql = {
    find: {
        before: (findOptions, args, context) => {
            // modify findOptions
            console.log(findOptions);
        },
        after: (result, args, context) => {
           // do something with result
            console.log(result);
            return result;
        }
    }
}

The definition of graphql-sequelize hook options can be found at https://github.com/mickhansen/graphql-sequelize/tree/a8bccd7f09f8a89aca689104800083f4e17e8ab4#options

Please see #59 for more details.

[alirizwan]

Thank you, I will test and merge.

[Nepomuceno]

👍