add --image-url-substitution

[roelarents]

In our use case we are mirroring external/public images (e.g. from DockerHub) to our own private container registry, so we don't have to rely on availability and rate limiting. But we only mirror particular/arbitrary tags. So version-checker will always see those as the latest tags. This PR adds a --image-url-substitution cli option. It enables to replace (sed-style) something in all image URLs, before looking it up. All our mirrored images are prefixed like privateregistry.example.com/mirrored/docker.io/original:tag, so we can remove that prefix.
Any override annotation is still applied after.