Make it possible to enable the extra plugins via conf.py
Moved the plugins to the plugin folder.

Include the new modules in setup.py.
Updated manual to include information about extra_plugins.
Lazy loading the plugin. Also Showing warnings when running planetoid with Python 3.
Make flake8 happy.
Merge branch 'master' of https://github.com/ralsina/nikola into first_class_extra_plugins
Updated.
Merge pull request #421 from okin/first_class_extra_plugins

First class extra plugins