Add Windows support

[ararslan]

The installer is written as a POSIX shell script and assumes it's running in a Unix-like environment. We could have a separate installer for Windows that's effectively a reimplementation in PowerShell or Batch. It would be fairly annoying to maintain two separate installers, though we could likely reuse a decent amount of what's implemented in AppVeyor.jl. That uses environment variables to construct the commands executed by cmd.exe which is a different approach than CirrusCI.jl takes. #14 will hopefully help significantly here by hiding the installation logic entirely from the user, I just need to finish it.