[metadata.tvdb.com] Added option to use IMDB ratings if it's possible

I don't know if it's the right place to pull for scraper updates. repo-scrapers has no pulls and here i found some old pull for thetvdb like https://github.com/xbmc/xbmc/pull/5272. If you need it in the scraper repo i can create a pull there.
This pull adds an option to allow retrieving IMDB rating if the tag IMDB_ID is present. If there isn't and the option is enabled it falls back to thetvdb rating.
Thetvdb ratings are pretty useless since usually they have less than 20 votes when imdb has hundreds or thousands.
I left the automatic fallback and not an option to chose because not all episodes and series have imdb ratings (especially the old ones).
It should be sync across the default / stock metadata addons. This avoids confusion and makes it easier for the users. Furthermore it should always default to the actual metadata source and use this as fallback in case the user activated an external source to get information from. By default it should not include external sources as this might slow down the actual scraping process. This is how i would do it.

I´ve not yet checked whether or not `metadata.themoviedb.com` handles it this way, but it definitely should.