Optionally verify presence of images referenced by digest

_(spawned from conversation in #18; thanks to @Zebradil)_

A user may want to verify that _digest_ references to images, in fact, resolve to an actual image.

Right now, when an image reference has already been resolved to a digest, `kbld` does not check for the presence of the image in the registry pointed to by the reference.

Allow the user to direct `kbld` to perform that check with a flag (e.g. `--verify-digest-refs`).

When this flag is set, `kbld` would:
- check for the presence of the image (e.g. a `HEAD` request for that image)
- include a note that the check was performed (what's a good experience? Perhaps a notice at the start of execution acknowledging the flag is set and that digest references will be verified. Perhaps a short note for each image that this check was successful.)
- even when the image is not present, report the error, but continue processing, and return a non-zero exit status.