Update 2048.tcl

I thought it would be fun to have a random play mode, which led to other ideas, including making it more game-like by clearing the screen between plays. That in turn led to a need to aggregate puts commands for speedier rendering over a high-latency connection. Also switched to event-driven input to allow user to pause random-play mode or toggle back to normal play. Added prompt to select board size, as well as a report of turns taken at end of game. Did more work on terminal control. Made the code more library-like so that a GUI skin might be feasible in the future.