Add per-plugin hooks, add docs about plugins.

 - Removes pre_hook, post_hook and ext_dir. With the latter being
   unused. Warns on startup if the hooks are present in config.

 - Adds IPlugin interface with pre_hook and post_hook methods.

 - Adds 'class' config parameter to plugins, which can be set to
   a class implementing the IPlugin interface, it will be
   initialized once on Mailman's startup, then before the DB
   setup the pre_hook will be run, after DB and other components
   the post_hook will be run. Plugin instances are stored in the
   config.plugins dict, with keys being their configuration section
   names.

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