Fix color output on Windows

Fix #128

I simply compare util_windows.go with util_unix.go, and setting c.Stdout, c.Stderr seems to have resolve this issue.

Screenshot

A question for this code: Returning stdout and stderr then io.Copy seems redundant, since setting c.Stdout = os.Stdout and c.Stderr = os.Stderr basically does the same thing. Is there reason for this?

@8LWXpg yes, the related code need to refactor 😄