Create easy builds for windows

[avahe-kellenberger]

https://ci.appveyor.com/project/grimfang4/sdl-gpu and https://github.com/grimfang4/sdl-gpu#building to build dynamic libs for the different platforms.

nim c -d:release -d:mingw --dynlibOverride:SDL2 --passL:-lSDL2 --dynlibOverride:SDL2_gpu --passL:-lSDL2_gpu --passL:-L. -o:platformer examples/platformer/platformer_example.nim would build the platformer example for windows, given that SDL2.dll and SDL2_gpu.dll were in the current directory.