This December, the Montreal-based airline will offer more than 20 direct flights to cities in the U.S. and Caribbean. As more Maritimers are looking to travel abroad, Air Transat is expanding its international flight service from Atlantic Canada starting next month. Beginning on Dec. 18, the Montreal-based airline will offer up to 20 direct flights per week to 10 destinations in the Caribbean and southern U.S. from Halifax and Moncton until the middle of May…