Feature Request: Work across forks

[cleithner-comcast]

A standard open-source project workflow on Github is to fork the project, push refs to origin, then create a PR across repos from origin to upstream. At present, stack-pr seems to only support one remote for both refs and PRs. It would be very nice if stack-pr supported this workflow, albeit I don't know how it would work since the branches would not exist in upstream. I hope whatever logic Github uses to track refs between repos would support the ability to use stack PRs across forks but I am not sure it is possible.