support single/batch image exporting to tar archive

[bgulla]

Ideally kim could be used to aid in the exporting of OCI images to a tar'd archive. This could be valuable to those wishing to transfer container images to an airgap network in a batch fashion. This could act as a replacement to the rancher-save-images.sh script when pulling rancher-specific images notated in a line-delimited file.

possible syntax:
> kim export --image-list=./rancher-images.txt --format gzip rancher-2.5.5-images.tgz

[dweomer]

Related to #4