Add prompt to input.
Fix tile spawning.

Specifically, spawn -two- tiles instead of one at game instantiation,
and randomly spawn tiles with a value of '4' now.

The spawn rate was choosen by starting 100 new games of the original
2048 and counting the number of '4's that appeared. After 100 games, 30
'4's appeared, giving a spawn rate of exactly 15%. Approximately ;).