[RFC] Add Postgres connection

When working with data stored in a database, it is not practical to load it via .CSV or .JSON files. It would be interesting to allow kepler to connect to a database and generate layers based on queries.

There are already [some issues](https://github.com/keplergl/kepler.gl/issues/85#issuecomment-399344986) related to it but I don’t know if someone is working on that.

I wanted to implement a fourth tab on the "Add Data To Map” modal so users could inform a connection string to the Postgres database and a query that would result in the layer.

I started working on my fork, I’m still struggling trying to add [node-postgres](https://node-postgres.com/) to the project, but any suggestions/comments are welcome.