Add pluggable components

Very much like !264 (closed) I am working on defining the concept of a 'plugin' in Mailman core.

In the end I want to have plugins be able to:

• Add Rules, Chains, Handlers, Pipelines, Styles, EmailCommands, CLISubCommands: done
• Add their own pre_hook, post_hook and exit_hook: almost done, looking for a good place for exit_hook
• Add functionality to the REST api, under /<api>/plugins/<plugin.name>/...: done