Which load average value will be shown in the mode line.
Almost every system can provide values of load for the past 1 minute,
past 5 or past 15 minutes. The default is to display 1-minute load average.
The value can be one of:
0 => 1 minute load
1 => 5 minutes load
2 => 15 minutes load
nil => None (do not display the load average)