Make dired-rainbow-define ignore case of EXTENSIONS

[legends2k]

    (dired-rainbow-define media "#de751f" ("mp3" "mp4" "MP3" "MP4"))

works but it'd be better, less verbose, to not need to define all extensions twice in both cases. I've not seen people differentiate files based on extension case so we want dired-rainbow-define to handle its EXTENSIONS parameter case-insensitive when it's a list.

[Fuco1]

This sounds reasonable. I'm surprised it even uses case matches, since Emacs usually ignores case by default. I'll look into it.