Get display brightness control file paths from config files

Paths can be defined as directories that must contain
both brightness and max_brightness entries or full paths to
brightness + max_brightness files.
Values in [modules/display] group are too appended, not overridden
When concatenating config values, put new data to front of the list
Merge pull request #22 from spiiroin/display_config

Display brightness control paths can be specified in config files