luci-app-statistics: add initial support for collect-mod-sensors

Due to a lack of a test environment this support only covers thermal graphs
so far. Please send the output of "rrdtool info /tmp/rrd/*/sensors-*/*.rrd"
if your system happens to support voltage, power or fanspeed sensors.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>