Add option(s) for controlling whether to log messages about keys not matching --path-filter

[jwodder]

While testing s3invsync, I frequently patch the code to eliminate log messages about keys not matching --path-filter in order to cut down on logging noise and the total size of the output. It would probably be a good idea to add a dedicated CLI option to turn this message off — or, alternatively, turn it off by default and have a CLI option to turn it on.