Multiple changes - Githubissues

A bunch of changes as I was trying to showcase your script in FreeNAS 11.3.

Added NETMASK parameter
Changed to default to a base jail - this is my preference, it seems 'cleaner' for long-running jails, revert at will
Clarified README.md around PlexPass version, and added a note about hardware transcode
Changed the default jail name to 'pms' from 'plex' - this is beyond bizarre, pkg would not install anything if the jail was named 'plex'. See forums, this may be my particular system, and it makes no sense to me, either.
Removed the $INTERFACE when creating the jail, this plays merry hell with 11.3-U1. Have not tested this changed way of doing things against 11.2. That's worthwhile doing, maybe the code needs to branch depending on version
Because of that pkg issue above, played around with how the pkg gets installed, and ultimately left it with the changes though they weren't the cause of things. It now installs from latest to begin with, instead of doing an upgrade - faster, less things get downloaded, users sees install notes. Can be reverted, if you hate the idea.

danb35 / freenas-iocage-plex