ikiwiki hosting internals/ todo/ ikiwiki setup should work on user machine

A user should be able to clone their site, get ikiwiki.setup from the setup branch, and run ikiwiki on it, w/o having ikiwiki-hosting installed.

Currently, this will fail because:

- The setup file uses a per-site home directory. How to deal with this? User can frob paths, of course.

- The setup file uses `add_plugins` to pull in some plugins that are not in stock ikiwiki. Maybe instead of listing those in the ikiwiki.setup, they could be automatically pulled in via a tweaked goodstuff?