Update instead of full download

[waszpaw]

Hi there. First - thanks for the script, useful.
Idea - would it be possible to add a flag / function to only check changes (as in rsync) instead of redownloading all the stuff? At first I though that's its default behavior but now I see it goes around again.
Thanks!

[andyjsmith]

It does loop through every image each time but it doesn't download already saved images, it just sees that it's already saved and skips to the next one, that's what this line does: https://github.com/andyjsmith/SmugMug-Downloader/blob/master/smdl.py#L102
However I do agree the resume functionality could be improved by skipping directly to the next file rather than going through everything again, I just don't have any free time to implement that. I'll keep this open, feel free to submit a PR if you or someone else want to make the change, could probably just add a resume.txt file that's updated with the last saved image or album.