Clean up and refactor configuration handling

Clean up and refactor configuration handling

Each "module" (CDB and CrackLib) now handles its own configuration
and setup, and the internal APIs are more straightforward and
simpler.

The plugin can now be configured without a dictionary, in which case
only the simpler checks available through the new configuration
variables are done.  This mode is mostly useful for testing, since
such simple checking can more easily be done via less complex password
strength configurations.