cron job configuration
- create a crontab file
- append cron entry to existing crontab
- replace crontab file
- add support for human readable
examples:
#add a crontab file $> rusty_cron add cronjobs.txt
#add a cronjob $> rusty_cron append "0 0 1 * *" "/usr/bin/run_some.sh"
#replace a crontab file $> rusty_cron replace newcronjobs.txt
#add cronjon in readable format $> rusty_cron append "everyday at 6am" "~/back-up.sh"