libj4status: Search environment config file in $XDG_CONFIG_HOME

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
j4status: Add "-c config" CLI parameter

Implement it as a simple environment setter

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>