Allow for scanning multiple scan targets at once

[wagoodman]

Very similar to anchore/syft#562 it would be great to specify multiple items to scan:

grype alpine:3.2 alpine:3.3 alpine:latest -o json
# scan 3 images

Or much like anchore/syft#1683 allow for scanning all images in an OCI index (for scanning all architectures):

grype my-image:latest --all-manifests
# scan 7 images references from the index