Their honeymoon got off to a dramatic start when the jumbo jetliner on which Mr. and Mrs. George Nunnermacker, who were married yesterday 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 this morning on a flight from New York to San Juan. Mrs. Nunnermacker is the daughter of Rev. and Mrs. William J. Leifield, who performed the wedding ceremony in St. John's Lutheran Church.