Windows support

[abhinav]

This adds support for using git-spice with Windows. I'm pretty sure it already works, but testing it is a bit difficult, especially since I don't have a Windows system to test on.

All terminal prompt use creack/pty to create a fake terminal. Until creack/pty#155 is merged, these tests cannot be run on Windows. We'll skip the prompt tests on Windows for the time being.

[abhinav]

This change is part of the following stack:

• 387 ◀

_{Change managed by git-spice.}