Added optparse-applicative to list of dependencies

Eventually we will be using optparse-applicative to give the ability to
have commandline arguments and flags
Removed extraneous checkGameOver

I was checking twice in a row if the game was over, which may have
caused performance issues.
Made generating negamax tree function more general

Moved the bulk of the generateNegamaxTree function to Negamax.hs instead
of being specific to Tictactoe.hs