Support for relational databases

First of all, great generator! Really fond of the structure and how it separates some concerns.
I'm thinking of always adopting this structure for my future APIs, but sometimes I need to use a relational database like Postgres. I managed to remove `mongo` and integrate `objection.js` and Postgres inside the same structure, could be a way to start.

Anyway, my question is, are there any plans to support relational databases? Maybe in a different branch?
I can work on a PR for this integration but first needed to know if it'd be useful.