Add flags for non-interactive use

Would be good to be able to schedule in batch without requiring user input. This could be implemented as an exit in case of the regular interactive prompts (non-unique filenames, not enough filenames for existing urls, or overwriting the targeted image download folder). In this case, it would likely be a good idea to check all three before exiting so the user can fix all at once instead of finding more issues on resubmit.

@johnbradley, as this was your request, I'd very much appreciate your opinion on the implementation.

No files, tests, or CLI entry points are named in the issue. Start by locating the interactive prompts for duplicate filenames, insufficient filenames for existing URLs, and overwriting the target image folder. Done means batch use can avoid input, checks all three conditions before exiting, and reports the problems together.

Written by the indexing model from the issue text.