golf out shell dependency

[jfhbrook]

After closing #1 and #6, the only external dependency I should have remaining is a working shell. One idea is to try replacing that with something that uses https://github.com/mvdan/sh .

This is definitely a stretch - implementing a whole ass shell is a lot of work, and non-Windows environments will definitely have one. Even in Windows, you could configure things to use Git Bash.