docs: def trim(self, video_name, out, start, duration):
          """
          Trims a clip to be duration starting at start
          @param video_name : name of the input video
          @param out : name of the output video
          @param start : starting position after the trim
          @param duration : duration of video after start
          """
          command = ['ffmpeg', '-ss', start, '-i', video_name, '-c:v', 'huffyuv',
                     '-y', '-preset', 'veryslow', '-t', duration, out]
          if self.verbose:
              print 'Trimming {0} into {1}'.format(video_name, out)
              print ' '.join(command)
          call(command)