`extern crate` with `Rust 2018` mention in README might be misleading for new users

[A]

Replace extern crate colored; // not needed in Rust 2018 by 2018+
New users could believe that the 2018 version exclusively don't requires it

[Alexis "Horgix" Chotard]

@AuracleTech I just opened #113 to fix this, can you take a look at it and approve it if it looks OK to you? 🙂