Make .kopiaignore compatible with .stignore (syncthing project)

There is well known and well established project syncthing designed for file synchronization between computers and it’s also written in GoLang. The syncthing project using well designed processing of “Ignoring Files”.

My suggestion is to barrow processing of .stignore into .kopiaignore since it is more feature rich in sense of ignoring patterns.

Unfortunately golang isn’t my area of expertise to attempt to do it on my own to make PR, that’s why a feature request.