Add --overwrite flag to rewrite every metadata json files

[Montspy]

New --preserve flag for metadata.py.
It will only update CIDs in the metadata.json files that exist.
If the json is not valid, it will print out an error, save a backup of the file (add .bak suffix) and regenerate a new metadata json file

[Montspy]

This implements #19

[Montspy]

@sk33z3r Should we make the default behavior to preserve metadata and have an --overwrite flag instead?

[sk33z3r]

Should we make the default behavior to preserve metadata and have an --overwrite flag instead?

Seems to feel more "user friendly" if you automatically overwrite so nothing is ever stale. I can't come up with an argument where you would want to keep the CID list stale like that, but I'm sure there is a use case I'm not thinking of.

At the end of the day, I make a change to a file and try to mint, I don't expect to have to regen CID because it's sort of a hidden feature to the user right now. Might not be completely obvious that the CID calc was a step in the process.

[Montspy]

Agreed, in all cases, running metadata should always "refresh" at least the CIDs.
a) Current behavior is to overwrite the whole metadata file. Including any manual edits.
b) My old proposal (--preserve) was to keep this as default behavior. The --preserve flag would only update the CIDs nad keep any metadata edits.
c) My new proposal (--overwrite) is to change the default behavior to refresh only the CIDs and --overwrite would overwrite the whole metadata file.

I believe behavior c) to be less prone to data loss for users who manually edit their metadata after generation, while preventing stale CIDs for all users

[Montspy]

Changed the default behavior to only update CIDs
Added --overwrite flag to force rewriting new metadata json files

[sk33z3r]

I believe behavior c) to be less prone to data loss for users who manually edit their metadata after generation, while preventing stale CIDs for all users

Yea, I agree with that and a good move to make it default with this option.