Update package.json

This adds the auto-update fields, but has the following problems:
- This is not an actual release, yet.
- The upstream does not include minified versions of the css/js, and probably won't.
  - Would this not be a good, and relatively easy, thing to add to the auto-update script? It seems like opt-in of, say, `uglify`, `clean-css` would both be one-liner kind of things, and would save many maintainers effort.