Rationalize the plugins:

* Rename on_terminate to on_generate.

* Add new on_db_create + on_db_load plugins and use them.

Update documentation to match.
Updated themes.

We've dropped the reference to /vim.css, and added support for
copying static resources which go with themes.

This updates #10.
Added to document themes.

This updates #10.
Renamed.
Renamed.
Load new template.

We use `tag_index.tmpl`, not `cloud.tmpl` as that makes more
sense.