Add credits detection functionality

[Brett824]

Suggested in #86

Tried to avoid duplicate code where ever possible, but I didn't get too clever because I wanted to just get the functionality working

Some notes:

• Factored the intro checking logic in PlayerManager.update to PlayerManager.check_intro_or_credits just for visual clarity
• Similarly factored some common logic into new PlayerManager.skip_marker function to implement PlayerManager.skip_credits
• Added credits skip functionality wherever intro skip existed (seeking, media keys, etc)
• Added two new options for credit skip prompt and always skip credits + added those options to the readme