LONDON (Reuters) - The Spice Girls took to the stage on Tuesday after the world premiere of a new musical loosely based on their meteoric rise to fame in the 1990s, earning huge cheers from an audience that only really got going at the encore. "Viva Forever!" was the brainchild of producer Judy Craymer, whose "Mamma Mia!" musical based on the hits of ABBA has earned nearly $2 billion worldwide and spawned a hit movie starring Meryl Streep. ...