[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).
themoviedb scraper doesn't fallback actually (probably should be fixed) but I wasn't talking about removing the fallback. Only about the option string.
It's ok for themoviedb to use "Get Ratings from" where 99.9% have imdb ids so only in very few cases it  doesn't use them but in thetvdb if users chose "IMDB" as rating they will get a lot of ratings from thetvdb anyway so they might ask "why? I selected IMDB ratings".
Naming it "Use IMDB ratings if it's possible" helps user to understand better that  they won't always get imdb ratings. 
If you still think it's better to use "Get Ratings from" it's ok and I'll fix it ;)