first plugin: size
PluginError exception
help info and argument parsing for plugins
execute plugins if are given
change plugin pattern name form ffplugin_NAME to ffplugin_test_NAME
changed short description of ff
shortened help about plugins - left only usage info here
small help improvements
added --help-plugins argument
misletter
added help to plugin 'size'
fixed recognizing condition '='
allow multiplier to be written in uppercase
better help for help-plugins
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
search for plugin in ~/.ff/plugins instead of ~/.ff_plugins