Code review comment for lp:~stefanor/ibid/rfc-plugin-330864

I'm sure I'll get complaints with my approach on this one, but one has to start the discussion somewhere :-)

* Thoughts on the caching interface? Surely the cachedir should be specified globally, but the API doesn't have any convenient hooks for that that I can see.
  - In that case, plugin cached-files should probably go in /cachedir/plugin/file.foo

* The search engine ain't so great. Anyone want to write something awesome?