Make commands objects instead of functions calling each other.
Now there is '?' command that can display command list and help about
individual command. Also the command handling is more distributed into
places where it belongs.
Bsnes spews quite a lot of warnings with -Wall (not that surprising,
given the looks of the code), but at least non-bsnes parts shouldn't
give warnings. Oh, this also fixed couple of bugs in Lua support.
