z plugin: allow usage while in sudo mode

$HOME directory is kept in my sudo environment and I'm able to use all my usual commands, scripts and settings after sudo -s. Z plugin makes some checks and terminates if user is not owner of _Z_DATA file.
This pull request adds _Z_OWNER variable. When it set to your main username (_Z_OWNER=dan for example) it's possible to use the power of Z while rooted.