Allow passing xor file path as arg

[nickgirardo]

Hi! I'm making this change to make testing my game's save capabilities easier

Since I don't really write C++ I'm sure there's a ton of jank going on, definitely open to all critique!

A couple things:

• Idk if the terminology you'd prefer is "xor file" or "flash file"
• I'm not freeing the string I create with strdup. I don't think this is too bad since it's only allocated at most once in the program's runtime, but I still don't like it lol. I'm aware C++ has some sort of destructors that might be used to clean things like this up, but I'm not really sure about what their deal is.

[clydeshaffer]

It's useful so I'll merge the PR with the caveat that it might get replaced with an numeric-indexed system for creating savegame slots, which will be part of eventual savestating features.