Adds a RecursiveFilterIterator

Fixes an issue where the Iterator would fatal error on .git folders. Version control and IDE folders aren't desirable anyway. So this ignores them from the filesystem scanner

We would need to also hide these from display on the BackUpWordPress admin page.

Fixes https://github.com/humanmade/backupwordpress/issues/592 for me
Do we need to check `is_dir`? Wouldn't we also want this to match `.git` files (which are often stored in submodules etc)?