* Add wrappergroup config option, which can be used to cause wrappers

* Add wrappergroup config option, which can be used to cause wrappers
  to be created owned by some group other than the default. Useful
  then there's a shared repository with access controlled by a group,
  to let ikiwiki run setgid to that group.
* ikiwiki-mass-rebuild: Run build with the user in all their groups.