Spirit Airlines unveiled plans on Tuesday to tap into a growing demand for premium travel as the no-frills carrier chases high-margin revenue to mitigate cost pressures and boost its earnings.The Florida-based ultra-low-cost carrier will offer intra-Europe style business class seats with a guaranteed blocked middle seat. The seating option includes one carry-on bag, one checked bag, priority boarding, and a snack and non-alcoholic beverage.The airline has been losing money despite booming travel demand, rai ...