Enable optional Parallel runs

[sledigabel]

at the moment all prs commits etc are sequential.
For large numbers, adding parallel processing could improve the user experience by accelerating the feedback loop.

[rnorth]

Yeah, I can see this being useful!

I think we'd need to be careful about two areas: logging and load on SCM/CI systems.

With logging it's fairly obvious; we'd need to present the logs in a non-confusing way, which is harder when parallelism is introduced.

Re load on SCM/CI systems, I think we should probably think about which commands create which kinds of loads. e.g. create-prs is already an operation which can be heavy on shared CI systems. The other operations that interact with GitHub will create different pressures, which are unlikely to be spread well for people using GitHub Enterprise.

The foreach command seems like an obvious candidate for parallelism without any surprise load concerns, though. We'd just need to think about the others!

[rnorth]

Doing #7 would help with this one.