Add support for GL_ARB_shading_language_include. Usage is identical to
the way GL_GOOGLE_include_directive currently works glslang (since
GL_ARB_shading_language_include is inherently a runtime feature and
glslang is an offline compiler).
Users can simulate their runtime environment by using a custom
glslang::TShader::Includer or using filenames that match their GL
runtime names.
Closes#249.
