minifyCSS noAdvanced option is more safe default

- `broccoli-clean-css` (minifyCSS) is using `clean-css`
- adding source where to look up for other `clean-css` options

My minified CSS wasn't working until I've set `noAdvanced` option to true. I purpose at least documenting where users can find documented options but I'm not sure if it's better to have it directly in source or on documentation site. For developers I think it's easier and faster to have it in source.