PR auto-merge flag

**Is your feature request related to a problem? Please describe.**
We'd like the safety and communication benefits of using Pull Requests while reducing the burden as much as possible

**Describe the solution you'd like**
I'd like a switch when calling git-xargs that will turn on the GitHub auto-merge PR feature on the PRs that it creates.

**Describe alternatives you've considered**
I guess we could use a GitHub action that does it somehow but then we'd need to roll that out everywhere too (there is obviously a nice tool that can do that :))

**Additional context**
Unfortunately the GitHub v3 API doesn't appear to expose the auto-merge feature, only the GraphQL v4 API does, so you'd need to use a library like https://github.com/shurcooL/githubv4 since go-github doesn't support v4 API.