Control backlight on various modern Intel(R) GPUs.

See here for a list of supported chipsets:
https://github.com/grembo/intel_backlight_fbsd/blob/master/intel_chipset.h

See here for the original project this has been taken from:
http://cgit.freedesktop.org/xorg/app/intel-gpu-tools/

Usage:
  $ sudo intel_backlight
  current backlight value: 30% (281/937)
  $ sudo intel_backlight 50
  current backlight value: 15% (141/937)
  set backlight to 50% (469/937)
  $ sudo intel_backlight incr
  current backlight value: 50% (469/937)
  set backlight to 51% (478/937)
  $ sudo intel_backlight incr
  current backlight value: 51% (478/937)
  set backlight to 60% (562/937)
  $ sudo intel_backlight incr
  current backlight value: 60% (562/937)
  set backlight to 70% (656/937)
  $ sudo intel_backlight decr
  current backlight value: 70% (656/937)
  set backlight to 60% (562/937)
  $ sudo intel_backlight decr
  current backlight value: 60% (562/937)
  set backlight to 51% (478/937)