TAPACHULA, Mexico/GUATEMALA CITY (Reuters) - A group of more than a thousand Central Americans in Guatemala headed toward the Mexican border on Tuesday as the first caravan of migrants paused in southern Mexico on its planned journey toward the U.S. border. U.S. President Donald Trump has vowed to begin cutting millions of dollars in aid to Central America and called the caravan in Mexico a national emergency as he seeks to boost his Republican Party’s chances in the Nov. 6 congressional elections. The caravan, which has been estimated at 7,000 to 10,000 mostly Honduran migrants fleeing violence and poverty in...