Added support for giant bomb api and modified the readme

[ltctceplrm]

This is to replace mobygames for future users that don't yet have an API key since it is now no longer free, per #164. I added a note in the readme about it but mobygames can stay in the plugin for existing users since they're currently unaffected.

As this is another api that requires authentication it should probably only be added together with #161 so users can still use unauthenticated apis like steam.