generalized lights in the lit shaders

.gitignore

assets/materials/_default.yml

assets/materials/default.yml

assets/materials/subsurfaceScatter.yml

assets/models/cube.yml

assets/models/out.yml

assets/shaders/_default.glsl

assets/shaders/base.glsl

assets/shaders/subsurfaceScatter.glsl

assets/textures/_default.jpg

docs/config.example.yml

include/overkill/Config.hpp

include/overkill/Material.hpp

include/overkill/MaterialSystem.hpp

include/overkill/ModelSystem.hpp

include/overkill/ShaderIntrospector.hpp

include/overkill/ShaderProgram.hpp

include/overkill/ShaderSystem.hpp

include/overkill/Texture.hpp

include/overkill/TextureSystem.hpp