Their honeymoon got off to a dramatic start when the jumbo jetliner on which Mr. and Mrs. George Nunnermacker, who were married Saturday in Jim Thorpe, were riding was hijacked to Cuba. A bearded gunman carrying a bag he said was filled with explosives commandeered the huge Pan American World Airways plane early today on a flight from New York to San Juan. The plane was down half an hour in Havana, then flew to Miami. Aboard the jetliner were 359 passengers and 19 crew members, all reported safe.