Add -f -failed flag for approve option

Add -failed flag to approve option. It's not right to copy all the run_data images into references/pending folder and replace the old ones. We should only replaced the failed images. Once you hit thousands of images, it will cause big i/o performance issue. Why replace a previous perfectly working reference screenshot in the first place? :smile: Oh, and without the -f flag, it will be pretty bad if you do code reviews because reviewers wont know what screenshot is new and what is old.

Anyway, I dont know what direction you would like to take so I added additional flag instead of making it a default behavior.