Move i3 configuration into its own module.

We want to keep common.nix nice and clean and in the long term the whole
config should be more modular.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
i3: Factor out conky configuration for i3bar.

Now we're generating full conky config files and we now have a new
conky.nix, which generates shell scripts for the left and right screens,
that run conky with the right configuration files.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
i3/conky: Autodetect CPU load and temperature.

Device ordering is something that can vastly differ from system to
system, so we're detecting the available CPUs on conky startup and pass
the corresponding hwmon/cpu expressions to -t.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>