Issue description

Pull requests seem to become stale at a time when they are ready to merge. I see this happening in a number of highly upvoted open pull requests. This issue isn't so much for these PRs specifically, but rather the problem in general: how can we avoid PRs from becoming stale.

Should something like probot-stale be added to Nixpkgs, so that we can get reminder-comments when a PR is becoming stale?