[mod] engine torznab - refactor & option to hide links

- torznab engine using types and clearer code
- torznab option to hide torrent and magnet links.
- document the torznab engine
- add myself to authors

Closes: https://github.com/searxng/searxng/issues/1124


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>