plugin items must now have 'plugin_' prefix, and there is new item: plugin_descr
reqritten part with help for plugins

plugins are prepared for many types of plugins (now are used only test
plugin).
argument --help-plugins now is --help-test-plugins, and can show list of
plugins with short summary, or full help about given plugins.
additional info about --help-test-plugins in help about --test
plugins path describe also priority of plugins

plugins in --plugins-path are most important, next from ~/.ff_plugins,
and then from ff own directory
in plugin's module, renamed action -> plugin_action