Boss Attack

Regardless of the fact I only have 5/6 bosses so far (and some of those aren't even finished); Boss Attack mode is now in the game. I wasn't initially planning to have it 2P/Dual enabled; but I thought it would be strange to leave that out, especially as fun as playing with both ships is in the main mode. I've also rearranged the layout of the title menu somewhat, too, allowing the player mode (1P/2P/Dual) to be selected separately from the game mode itself. This should make adding additional modes easier as well.

As of now:
- Boss Attack is unlocked along with the Gallery and first secret ship by beating the game once.
- like the Enemy Test mode, checkpoints are temporarily disabled independent of them being active in the main game. No continues are given, either.
- Boss Attack has it's own separate hiscore table and end-stage tally
-- you are rewarded for less time taken (in place of wave clear bonuses)
-- perfect clearing the mode (1 life) gives you a HUGE multi-million bonus
-- remaining lives give you far more points than before, although bonuses are scaled the same way, with the amount you get per life depending on how many lives you start with.
-- hiscore table awards/medals are given for subsequently clearing bosses without dying, these are not reset when you die.