luci-app-statistics: CPU plugin: hide 'idle', tweak defaults

* Add functionality to hide the metric of 'idle' state
  from the chart and data table. Many routers are mostly idle,
  and the 98% 'idle' dominates the graph reducing its usefulness.
  Without 'idle', the smaller CPU usage spikes are visible.

* Hide 'idle' by default. Provide config option to show it.
  (note: the option in inside LuCI, and has no impact on actual
   data collection by collectd.)

* Tweak the defaults to use the percentage data by default.
  It makes more sense to average users than jiffies.

* Set the current LuCI defaults also in the config file.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>