Update NES implementation to use MMC5 mapper

[puzzud]

The MMC5 mapper has one clear advantage over any other: individual character / tile graphics can have a different palette entry association than its surrounding tiles.

I have a working implementation of this mapper in puzl6502 (Google Code)--it uses ACME.

This change is not trivial, as MMC5 has more requirements in terms of when PPU memory can be updated.

It's important to note that the use of MMC5 is not popular among NES homebrew, as standalone carts with this mapper are not manufactured. However, PowerPak (https://www.retrousb.com/product_info.php?products_id=34) does partially support it. I've tested it as well.

[puzzud]

I'm having second thoughts about using MMC5.

I could see people wanting to use this project's source code just for targeting NES with an expectation that they could create physical cartridges not requiring something like PowerPak.

As mentioned, this has an impact on how easy it will be to design & implement cross platform games (targeting NES, Atari computers, SNES, etc.).

[puzzud]

Ideally, it'd be good to implement MMC5 support as an optional platform. Therefore, it'd be indicated through a CFG file and alternate make platform. This notion also suggests that additional source files need to be made and existing files modified with processor switch statements.