fixed file descriptor leak

[ThrosturX]

Fixes issue #14 the easy way, leveraging the fact that cmd.Run() waits until the process has finished, thus closing the Stderr logger manually.

A possibly better fix would be the following, I guess it's up to the PO to decide.

writer := bufio.NewWriter(outfile)
defer writer.Flush()              
                                  
if err = cmd.Start(); err != nil {
    /* handle the error */
}                                 
go io.Copy(writer, stdoutBuff)

if err = cmd.Wait(); err != nil {
    /* handle the error */
}