Add command for setting a warm color depending on the time of day

Similar to apps like f.lux, Redshift or OS features like Night Shift, `lights` could provide a command for setting a cold color if it's daytime and a warm color otherwise. This might require specifying the user's location as an argument (once we have a config file as proposed in #1, perhaps there too?).

Having such a command would additionally make it easy to e.g. add a systemd timer or cronjob for automatically updating the light throughout the day (if it's on).