FR: Implement AsRef<str> for Painted<&str>

[ogarcia]

First of all, if you don't see it, just close the issue, because I may be asking you to complicate it too much.

The idea is that you can do something like this:

use yansi::Paint;

fn print_message<M: AsRef<str>>(message: M) {
    println!("{}", message.as_ref());
}

fn main() {
    print_message("Hello");
    print_message("World".red());
}