[ ] Add a config file
[ ] Use a better filter than a simple median, maybe a kalman filter
[ ] Add a way to customise the brightness curve
	Percieved brightness is not linear, on low light you need a slightly higher
	brightness and on high light you can get away with a lower brightness
	than the max, the goal is to find the right curve for the screen
[ ] Automatically detect backlight device, or at least make it configurable
[ ] Add signal handlers
	[ ] SIGUSR1 to reset the user offset
	[ ] Handle kill by closing all file descriptors