Multiple references in target

As discussed in https://github.com/michaelsauter/crane/issues/48, support for providing several group/container references as target, without (yet) switching CLI library. `-t/--target` is left for backwards-compatibility, but the migration path is so easy that I'd suggest that the support and the FIXME blocks should be cleaned up as early as 0.10.x (given that this is released in 0.9.x).