Add option --keep-existing that skips already downloaded files

[al-broco]

When the option is set, any files that already exist locally are skipped and not downloaded again. Files that exist locally but where the file size differs from the backup will be downloaded.

This can be used to resume a failed or interrupted backup. Personally, I have been using this to get around issue https://github.com/hackappcom/iloot/issues/35.

I hope this can be of use to anyone. Tested on Linux.

[hackappcom]

Thanks