Fix downloading filenames containing forward slashes

Hey I recently came on this project while trying to download some photos from one of my albums. If you want a repro case, here's one of my albums that contains several files with forward slashes in their name:
flickr_download -d 72157639469468893

I threw in a pretty hacky filename validator to avoid the crash. It works for my specific case but there really isn't a great solution when you have to yank out 1 or more characters from a filename. Anyhow, thought I'd throw this upstream. Thanks for the project!