Allow default base_url to be set for running cron tasks

Cron can send emails. And in some cases that email ends up having a link back to the site as "http://default/"

This occurs when a site doesn't have a $base_url set (and can't have one set). Add an option to set the URL that cron should run as.

Use the $options['variables']['base_url'] or $command_specific['cron']['uri'] = "" 

As potential candidates