[WIP] Plugins architecture proposal

[taganaka]

No description provided.

[taganaka]

cc @tmaier

[coveralls]

Coverage decreased (-0.62%) when pulling 02ee484 on plugins into 0e4b19a on master.

[taganaka]

API Proposal:

Polipus.crawler(name, starting_urls, options) do |crawler|
  crawler.register_plugin(Class::Name, plugin_options)
end

[tmaier]

1. Shall a plugin be able to be registered more than once?
2. Shall it be possible to unregister a plugin again?
   This way, we could register some default plugins. If someone does not like them, they can unregister them by themself.
   This would also be necessary to stay backwards compatible: We automatically register the plugins resulting from the old options. And give a deprecation warning.