Support for skipping preprocessing

[anstdelisle]

It would be nice to have the option of skipping the preprocessor. As an example, gcc does it with the -fpreprocessed: https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#index-fpreprocessed

This is useful for cases when the preprocessed file is held on for later and enables skipping the redundant preprocessor step when using a pattern like:

glslc -E -o FOO.i.frag FOO.frag
glslc -c -o FOO.spv FOO.i.frag