irssinotifier.pl: screen integration

add new option `irssinotifier_screen_detached_only` to (if we detect
we are running inside screen) only send notifications if screen is
detached.

The use case here is that you might use a screenaway script but don't want that script to mark you back immediately once you reattach, but you also don't want more notifications while you are attached but still marked as away.