Having the jekyll assets here is a mistake, because they need to be versioned
separately to the spec: we keep old versions of the spec, but they sometimes
need to be restyled to match the latest website.
Make the argument to dump-swagger.py be the name of the output file rather than
the output dir, and give it a sensible default.
Also, use the absolute path to the output file, so that the output path is
relative to the user's cwd rather than the templating dir.