#261 libretro port

[eadmaster]

I'd like to suggest adding a libretro port.

Advantages over the SDL version:
- custom shaders support
- standardized user-interface and hetkeys
- only requires OpenGL to compile (it could be more portable)
https://github.com/libretro/RetroArch/wiki/Compilation-guide-%28Linux%29
http://www.libretro.com/index.php/wiki/compilation/linux/