add flag to validator to ignore certain warnings

[pdurbin]

mlcroissant validate --jsonld croissant.json is extremely helpful but I would like to configure it to ignore certain warnings.

Perhaps we could add an --ignore flag like flake8 has. Here's an example from their docs:

flake8 --ignore=E1,E23,W503 path/to/files/ path/to/more/files/

(By consulting various lists of many possible codes you can see that E1 is about indentation, W503 is about line breaks, etc.)

I'm opening this issue because I see it as a possible solution for this other issue I opened:

• #609

That is, I would be happy to ignore the warnings about strings we use for dataset versions.

However, adding a generic --ignore framework is probably a good first step, so I'm opening this issue.