Add  RETRO_ENVIRONMENT_GET_SAVE_DIRECTORY

I undestand this should be fixed in the cores themselves but it might be complicated for some cores. Some cores save information in different places. I think one of the best points about RetroArch is standarization

This affects mednafen, mame, fba

https://github.com/libretro/RetroArch/issues/523
https://github.com/libretro/mame2013-libretro/issues/22

I already implemented this on mednafen. Maybe there should be a libretro_optional.h or something alike for this kind of features that might be deprecated in the future.