Added love.joystick.loadGamepadMappings (resolves issue #842), and Joystick:saveGamepadMapping.

The latter returns a mapping string, and the former loads a newline-separated list of mapping strings.

src/modules/joystick/sdl/wrap_JoystickModule.h

@@ -38,6 +38,8 @@ int w_getIndex(lua_State *L);
 int w_getJoystickCount(lua_State *L);
 int w_setGamepadMapping(lua_State *L);
 int w_getGamepadMapping(lua_State *L);
+int w_loadGamepadMappings(lua_State *L);
+int w_saveGamepadMapping(lua_State *L);
 extern "C" LOVE_EXPORT int luaopen_love_joystick(lua_State *L);
 
 } // sdl