Add a submodule for SDL-emscripten

Trying to follow http://gsathya.in/blog/mozilla/sdl2-emscripten.html
to see if I can get this example for SDL2 working with emscripten
*sigh* spent a long fucking time trying to get this to work w/ emscripten

Got close, but eventually had to give up.
Going to go try d3, I guess