Fix color output on Windows

[8LWXpg]

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

[8LWXpg]

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?