Enabled a plugin of the app directory

Install from app/config/*.py when it is not found in lib.plugin.

ex:
in config/settings.py
plugins = ['plugin_a', 'plugin_b']

in lib/plugin.py
class Plugin_aPlugin

in app/plugin/plugon_a.py
class Plugin_aPlugin

in app/plugin/plugin_b.py
class Plugin_bPlugin

This case, 'plugin_a' install from lib/plugin.py.
And 'plugin_b' install from app/plugin/plugin_b.py.

Give priority to lib.plugin now, but that a priority to app/plugin could
possibly be better.I think the user want it.