Changing instruction representation in anticipation of LLVM backend
Adding ability to generate LLVM and then assemble that into an executable

Subcommands have changed from
    python hugo.py run file.hugo
    python hugo.py compile file.hugo

to
    python hugo.py run file.hugo
    python hugo.py gen-c file.hugo
    python hugo.py gen-llvm file.hugo
Update README with new subcommands