[Request] Add an option to add just the missing packages

[leodotcloud]

Currently -u option is the only way to add the missing packages automatically but it also has the side effect of updating the existing packages. Many times it's useful to auto add the missing packages to trash.conf/trash.yml without upgrading the rest of the options. Requesting to --add-missing/-a or some other flag for the same.

[joshwget]

This would be a great feature 👍

[imikushin]

@leodotcloud Nice feature! PRs welcome ;)