colors are not shown if output is piped to `less -R` (unix)

[mattsawyer77]

I wonder if color support detection can be improved? Perhaps there would be a more robust method than using libc::isatty (which, if I'm reading it right, would be https://github.com/mitsuhiko/clicolors-control/blob/master/src/unix.rs#L5)? Being a rust novice, I don't have any constructive criticism, unfortunately.