Add option to update metadata JSON CIDs and final CIDs

[Montspy]

Users might want to edit images and/or non-critical metadata after generation.
That means CIDs can become stale and should be updated to the new file contents.
I propose we add some option(s) to metadata.py to update the image and animation_url CIDs as well as the final metadata CIDs.
It would run the normal flow but read JSON, update the CIDs and write back.

[Montspy]

This feature is implemented in #28

[Montspy]

Merged. Closing.