Make #913 link opening confirmation optional

### Is there an existing issue for this? - [x] I have searched the existing issues ### Problem Since #913, a confirmation dialog was added for opening all external links. As I said in https://github.com/pwmt/zathura/pull/913#issuecomment-4472142761, I find it slightly annoying to have to confirm each time I want to open an http link in my web browser for instance, and for my use case the benefit is definitely not worth the cost; from the reactions to this message it seems at least two other persons agree. ### Expected behavior Would it be possible to add a configuration option for skipping the confirmation dialog altogether when opening external links ?