Added group creation option and corresponding documentation

[courtarro]

Added a new config option SSH_GROUPS, which is a list of groups and GIDs to create. These groups will be created before the users are created (assuming SSH_USERS is also specified). Specifying this option disables automatic group creation of user-named groups if you also specify SSH_USERS since it's assumed you want greater control in this case.

My use case is such that my users are all members of one group, so this lets me create the group first, then create the users and set their GIDs to the group I just created.