Add support for crontabs provisioning and custom nginx-server settings through Homestead.yaml.

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: '*'
```

And extend **nginx settings** like this: _(I needed to boost **client_max_body_size** to **100M**)_
```
sites:
    - map: homestead.app
      to: /home/vagrant/Code/Laravel/public
      client_max_body_size: 100M
```