Plugin config file

Each plugin can include a file  in its root folder.

All config files are read at superadmin login. The configuration is only applied if the version in the configuration file is higher than the version in the database. PHP function version_compare is used to compare versions.

Suggested fields

 * deactivateable, if false, the button for deactivate will be hidden
 * Settings fields could be moved to config file, as some other fields. BUT: Localization?