Draft: Add tmp and public as caches

We might be able to speed up nanoc compiles by caching the nanoc tmp files and the compiled files, so it doesn't rebuild everything every time.

I've been meaning to dig into this for a long time, but shelved it as things got too busy and it wasn't a priority. Now I'm hoping to do a bit of cleanup while I have some time.

Related to https://github.com/nanoc/features/issues/57