Improve loop

One pass through the loop takes one story from todo to done:

pick story -> plan -> code -> ./build.ps1 -> self-test -> snapshot -> compare -> fix -> commit -> update STORIES.md

Stop after the milestone the user named, or after the current milestone if they named none. Then report.

1. Pick

Open docs/STORIES.md. Take the first todo story in the current milestone whose dependencies are done. Set it to doing.

2. Plan