Made rebuild.py more modular
Made rebuild.py more modular

As with 44d08ced6ee3ede94fb45a8859ff301f00934b4f, this is being done
because I might do a GUI wizard eventually.  Additionally, I'm planning
to merge rebuild.py's functionality into fp-assets.py so that the UI is
simpler and the command-line arguments can more closely mirror those used
by archivers such as tar.

snickerbockers/fp-assets