Add support for adding cron jobs through Homestead.yml.

Make it possible to **define crontabs** and getting them **installed for 'root' on VM**: _(minute/hour/monthday/week/weekday defaults to '*' if left out)_
```
crontabs:
    - command: /home/vagrant/Code/jobs/addresses-import-job.sh
      minute: '*/15'
      hour: '*'
      monthday: '*'
      week: '*'
      weekday: '*'
```