Changed regex to insensitive, and added a function to retain the case of the original source.
Changed regex to insensitive, and added a function to retain the case of the original source.

m1c0l/xkcd-substitutions,Posnet/xkcd-substitutions,Posnet/xkcd-substitutions,piquan/xkcd-substitutions,borndangerous/xkcd-substitutions,m1c0l/xkcd-substitutions,borndangerous/xkcd-substitutions,piquan/xkcd-substitutions