Feature Request: Private registries

[Libbum]

I'd like to be able to add a private registry into the pipeline with something like

Pkg.Registry.add(Pkg.RegistrySpec(url = "git@github.com:MyCompany/JuliaRegistry.git"))

and an appropriate way to use my ssh key to authenticate.
As an alternate, https://${GITHUB_USER}:${GITHUB_TOKEN}@github.com and some environment variables should also be supported I guess.

Would be happy to have a go at adding this myself if you could point me to the best place in the code this would sit.