this correctly makes them accept 1 argument, and auto-completes the
comma-separated list of lint flags

Don't spew a warn!() in reset() if num_colors is 0, because
non-color-supporting terminals are legit. Use debug!() there instead.
Continue spewing warn!() if we believe the terminal to support colors.

Use a better warning when the `op` capability can't be found.

To match Rust conventions and enable use of `do` etc, make sure the
closure is the last argument to the `new` method.