Make it possible to enable the extra plugins via conf.py
unbreak monospace when using nocdn
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
Merge branch 'master' of https://github.com/ralsina/nikola into fix4sergey