make RL format detection more liberal

For my actual corpus, I need a somewhat more liberal approach to detecting LP's:   accepting "disc", not just "sound disc", and accepting "disc" and "33 1/3 rpm" anywhere in the 300, not just in the 'correct' subfields. 

I don't _think_ this will result in very many false positives, but that would be the risk. 

If you can't accept this more liberal algorithm into your code (or don't even have time to figure out if you can!), just let me know, and I'll just have to fork and/or write my own code based on yours instead of sharing the gem, so it goes sometimes, no biggie.