Windows is not supported

[LynnKirby]

The commonmark executable does not run on Windows because it reads /dev/stdin as a file. The project also uses Make and various *nix commands for development instead of standard npm scripts.

[jgm]

I don't develop on Windows. If you want to submit a PR that allows the executable to work on Windows, it would be welcome. I'd also consider a PR that removes use of Make, though I can't guarantee I'd accept it before I see the alternative.