Add a 'plugin' mecanism

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Some features can be moved into 'plugins' to allow better option management.
For example the password checker who require CrackLib can then be available as an option.
This would be useful for user with CrackLib not available on their OS.

We have to check first for available mechanisms like bag.web.pyramid.plugins_manager,
the pyramid extensibility feature, or just make our own plugin mecanism.