search for plugin in ~/.ff/plugins instead of ~/.ff_plugins
use _import_plugin instead of __import__
few code comments
docs for _parse_input_args__plugins__*
plugins system rewritten to OOP

every plugin module is wrapped into FFPlugin object. Plugins collection
is available via FFPlugins object. There are static methods: find (previously
function _parse_input_args__plugins__help) and find_all (previously
function _parse_input_args__plugins__list).
Import of module is done in FFPlugin itself.
run tests vie FFPlugin.run
changed order of arguments for plugins action