Removed "there is a bug" comment; no bug (except awkward game end).

I had thought the black king could move up one space; I did not see at
first that that last square was under attack from a white bishop.
Added game-end detection.  Misc cleanup.
Added rules for Antichess.  Fixed bug in pawn move generation.
Added feeble support for pawn-promotion (queen default instead of checking other options).
Added arguments to select chess or antichess.