nixos: Add option services.cron.cronFiles

This allows you to configure extra files that should be appended to your
crontab. Implemented by writing to /etc/crontab when the cron service starts.
Would be nicer to use a cron that supports /etc/cron.d but that would require
us to patch vixie-cron.
Merge branch 'nixos-cronfiles' of git://github.com/rickynils/nixpkgs

nixos: Add option services.cron.cronFiles