Disable colors by default when being run under Windows

#267 added the `--no-color` argument but colors should be disabled by default on Windows, otherwise you end up with messages like this:

```
D:\Webserver\htdocs\plugin-development\wordpress>wp plugin activate akismet
←[32;1mSuccess:←[0m Plugin 'akismet' activated.
```

*Hope I did this right -- this is my first contribution to WP-CLI and my first pull request in a very long time. :wink:*