HONG KONG, Sept 13 (Reuters) - The leader of a rebel group in India's northeast challenged the world's biggest democracy to live up to its name and let the people of the troubled state of Manipur choose for themselves if they want independence. Sanayaima, chairman of the United National Liberation Front (UNLF), said there was no room for peace talks with New Delhi without U.N. mediation, nor any middle ground short of a plebiscite on the restoration of Manipur's "sovereignty". The UNLF was established in 1964 and has been waging an armed struggle since 1990 for independence for nearly two...