add NEZplug++ emulation core as an option for Gameboy

[djmaximum17]

moved from. ValleyBell/in_vgm-libvgm#2
it would be nice to have NEZplug++ as an option in the emulation core settings for Gameboy.
sound emulation is more accurate, it emulates volume zombie mode, which some GB games like "Prehistoric Man" and "superman" uses.
http://offgao.net/program/nezplug++.html

[ValleyBell]

I'm not sure about that "zombie" mode, but MAME's GameBoy core emulates a bunch of oddities, including wave RAM corruption. (Especially wave RAM corruption happened to break a few GBC games, since it happens only on mono GBs.)

Thus this will be low-priority for me right now. (PRs will be accepted, of course.)

[djmaximum17]

hay, sorry it took me a year, too many projects got in the way. but I finally got around to do some test vgm logs of the Prehistorik Man soundtrack. it sound correct in nezplug++, MAME does not emulate this volume trick correctly. gbs file is also included.
Prehistorik Man.zip