Move sg.config.yaml to repo root & allow private overwrites

Created by: mrnugget

This moves the sg.config.yaml to the repository root so that one can just use sg start and sg run gitserver etc. without having to specify the -conf flag.

It also adds support for a local .gitignored sg.config.overwrites.yaml file that users can add to the repository. If it exists, it will then get merged with the basic configuration when sg starts.

What's the usecase? Custom command sets, custom env vars, etc. that you don't want to commit to the repository.