zsh: Move into its own module.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
modules: Add "git" module for system-wide config.

Allows to declaratively specify the Git configuration options using
nested attribute sets.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
common-workstation: Switch to vuizvui Git module.

Yay, no .gitconfig anymore :-)

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
modules/zsh: Fix formatting and check for enable.

We don't want to have the zsh module active for _all_ machines, only for
warkstations.

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