Keep color handling out of WP_CLI::line()

![spanish inquisition](https://f.cloud.github.com/assets/225715/646401/9f9da10c-d3cb-11e2-8eb6-272bb60971f2.jpg)

The coloring abilities are only used in a handful of core subcommands (plugins, themes and `core version`).

If we don't fix it now, before 1.0, it'll never get fixed.