mia: allow atari 2600 to load .bin format roms

mia/medium/atari-2600.cpp

@@ -1,6 +1,6 @@
 struct Atari2600 : Cartridge {
   auto name() -> string override { return "Atari 2600"; }
-  auto extensions() -> vector<string> override { return {"a26"}; }
+  auto extensions() -> vector<string> override { return {"a26", "bin"}; }
   auto load(string location) -> bool override;
   auto save(string location) -> bool override;
   auto analyze(vector<u8>& rom, string location) -> string;