Spirit Airlines is introducing a new fee, but it’s for an option the carrier thinks customers will be happy about: in-flight Wi-Fi. Spirit announced Friday that it will become the first North American no-frills “ultra low-cost carrier” to add that capability, unveiling an aggressive plan to equip its entire fleet of about 120 planes with Wi-Fi by next summer. Spirit touted the move as fitting into a broader strategy to “invest in the guest” and its ongoing effort to improve customers’ experiences with the carrier.