Add color support to env_logger

[lfairy]

On Unix, we can use the ansi_term crate or roll our own. I'm not sure how it works on Windows; we can just skip colors there for now.

As for configuration: we can add a new environment variable RUST_LOG_COLOR. It will take the values never, always, or auto (default). If auto, the logger will use color only when stderr is connected to a terminal.

[FredrikNoren]

👍

[calebmer]

Color support along the lines of ansi_term would be very nice.

[antoyo]

The pretty_env_logger crate supports this.

[KodrAus]

We've got sebasmagri/env_logger#2 in env_loggers new home to discuss this so I'll go ahead and close this one now.