Consider inverting the behavior of -update and -update-only

[emidoots]

I think -update-only may be a lot faster in general because it doesn't need to shuffle files around in order to remove unused .golden files (although that should be fairly cheap, it could add up with many files/tests on large code bases)

So it may make sense to have -update not remove unused .golden files, and have an additional -cleanup flag supplied instead. This could also prevent confusion like https://github.com/hexops/autogold/issues/1