luci-app-statistics: add options for graph series sorting and color

luci-app-statistics: add options for graph series sorting and color

Add two data series-level options "negweight" and "posweight" that
will use the numerical value of the data instance name
as the sorting factor. This enables e.g. sorting by CPU frequency
in the cpufreq module.

Add new graph-level option "ordercolor" to use colorsfrom a
pre-defined color table for the data series. This enables keeping
similar colors for similar data series in different plugin instances.
E.g. CPU frequencies in several cores that are handled and displayed
separately.
 (note: the table has 8 items and if there are more series, it uses
  the same colors again. The table can be easily extended/modified)

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