Added ALDRYN_SEARCH_EXCLUDED_PLUGINS setting

[ghost]

This setting excludes specified CMS plugins from getting index data. This setting is really helpful when we use plugins that are not compatible with aldryn_search such as: djangocms-audio, djangocms-video. This setting will also available for modules like djangocms-blog, that uses get_plugin_index_data from this aldryn_search addon. Please, accept pull request

[ghost]

Thanks for review, @czpython, I've already done mentioned requests

[czpython]

Thanks!