on provided prg/chr rom input args for dumping.  Mirroring is sensed &
entered for fixed mirror mappers.  So this is basic iNES file format.

Still don't have automatic banktable locating, nor mapper detection.

But this provides a basic header that should work with most currently
supported NES mappers.  If the headers need tweaked, I recommend opening
in Mesen and using it's header modifcation tool.