utilize sw.Target rather than hardcoded os.Stderr

[bitspill]

StandardWriter.Target is set[1] but not used and instead it's currently hard coded to os.Stderr rather than using the os.File that was passed in through NewStandardOutput(file *os.File)

[1] https://github.com/azer/logger/blob/8b7d1ee/standard-output.go#L16

[azer]

Thanks!