Make Importer architecture pluggable

[julianpollmann]

Right now Non-RSS Content cannot be imported. It would be nice to also handle content sites which don't provide a rss feed (e.g. ARD Audiothek)

It would be nice to have a pluggable system for Importers so that it's not limited to RSS and additional Importers can be developed. A further (pluggable) Importer could possibly ingest a url and crawl subpages to extract media/posts. E.g. ARD Audiothek provides schema.org metadata at each media page, but no rss/index.

If multiple Importers are not implementable a custom/external RSS feed can be developed for these sites.