pt-stalk external command interval

From bug 1190636:

Looking at the code:

   if [ "$CMD_VMSTAT" ]; then
      $CMD_VMSTAT 1 $OPT_RUN_TIME >> "$d/$p-vmstat" &
      $CMD_VMSTAT $OPT_RUN_TIME 2 >> "$d/$p-vmstat-overall" &
   fi
   if [ "$CMD_IOSTAT" ]; then
      $CMD_IOSTAT -dx 1 $OPT_RUN_TIME >> "$d/$p-iostat" &
      $CMD_IOSTAT -dx $OPT_RUN_TIME 2 >> "$d/$p-iostat-overall" &
   fi
   if [ "$CMD_MPSTAT" ]; then
      $CMD_MPSTAT -P ALL 1 $OPT_RUN_TIME >> "$d/$p-mpstat" &
      $CMD_MPSTAT -P ALL $OPT_RUN_TIME 1 >> "$d/$p-mpstat-overall" &
   fi

You can see that all commands have an interval time of 1 second.
When collecting data for a longer period of time (like for example 1 hour or 1 day), it would be good to be able to define that.