Allegiant follows lead of Spirit Airlines
                                
                                
                                    (Newser) -
                                    Uh oh, travelers, this might be turning into a trend. Starting tomorrow, Allegiant Air will begin charging passengers up to $35 for their carry-on bags,  MSNBC  reports. Passengers can bring one carry-on aboard for free, but only if it can fit under the seat in front of them, the president...