add windows support

[rizrmd]

Please, please add windows support.. currently when i use it in window, it throws:

D:\Go\src\github.com\go-cmd\cmd\cmd.go:204:9: undefined: syscall.Kill
D:\Go\src\github.com\go-cmd\cmd\cmd.go:279:48: unknown field 'Setpgid' in struct literal of type syscall.SysProcAttr

[daniel-nichter]

Sorry, no plans to support Windows (re issue #2, #5, and #12). This code use low-level Unix-specific code, so it's not clear how it would work for Windows (and Windows programming is not my expertise).

[daniel-nichter]

Windows should work as of v1.1.0.

[rizrmd]

😍