Allow configuration of startup information and process creation flags

[Taraflex]

[murrayju]

Thanks for the submission @Taraflex! As is mentioned in the README, I'm not in a great position to vet this, as I've long stepped away from Windows development. Before merging, I'd like to get a review from some other contributors, like @MV10, @0x53A, @mmiszczyk, or @KamenRiderKuuga.

It would also be helpful if you could add a description to this PR, to better explain the what, why, and how for this change.