Screen backlight brightness

Feature gets value of acpi backlight value and sets user defined value.
By default, I decided to leave configurations values commented as in
testing.  I've known a good number of people to use this with success
but am unwilling to attest to just yet.  I have tested the setting and the
screen does dim on battery and return to set value on ac.  From the
ThinkWiki I've read that this can save a decent amount of power.