Support for beginning a transaction inside boiler generated package

[eduacostam]

This is only a proposal, maybe it needs some discussion.

The idea is that if we use boiler.SetDB(db) method in order to be able to use the G variants of the models, and avoid having to pass around a sql.DB instance, then there's no way to create a transaction inside a function of a struct that does not have the db instance in scope.

It might be nice to have a method in the models package func Begin() (*sql.Tx, errors) that uses the db instance passed into the SetDB method for creating a transaction and returning it, this way we can use transactions (for example when working with relationships) but still avoid having our structs have the need of the DB instance.

[eduacostam]

Nevermind, I've just found that there is indeed a Begin() method that returns a Transactor... I don't know how I missed it before...

[aarondl]

Will document better.

[eduacostam]

That's awesome @aarondl, much clearer now haha
Thanks!