Substring coloring

[apohllo]

Your gem is really good, as it doesn't force users to extend String class. I had a similar solution which was bad in this respect (see apohllo/colors). But it has a feature, which is missing in your gem - I can color a substring within larger string, eg. "quick brown fox".red("fox"). It would be great if this feature were implemented.

I am going to fork your project, to play with that idea.