Multi-cat List plugin for Seditio CMS.

This plugin lets a page belong to multiple categories and be listed in matchable
categories.

Usage:
1. Install the plugin.
2. Choose and fill the field of sed_pages table that will be used as the list of
extra categories. The default one is page_extra4.
3. Add the proper EXTRA tags to your page.add.tpl and page.edit.tpl, where users
will list category codes for the page (besides the main category selected with
dropdown as normal).