Add --exiftool-skip option to export

> Thanks for the feedback. It's useful to hear about a new osxphotos user experience and also some of these edge cases.
> 
>  However, I want to use exiftool on everything else.
> 
> I should add an option to allow exiftool to be skipped for some files as there are other cases where exiftool causes issues. Perhaps `--exiftool-skip ".CRW" --exiftool-skip ".BMP"`. Will open a new issue for this.
> 
