Allow proper rebasing

[matthieusieben]

This plugin should specify the relativeTo and target clean-css options in order to allow proper rebasing.

[seven-phases-max]

Well, the plugin is only wrapper of clean-css, it does not really knows anything about CSS code it passes through, so if clean-css itself does not provide these options then the plugin can do nothing about it.

[matthieusieben]

Please see target and relativeTo

[seven-phases-max]

I see. (Now I wonder why CLI options are hardcoded by the plugin tinsel at all. It seems to be quite tedious to sync https://github.com/less/less-plugin-clean-css/blob/master/lib/parse-options.js to https://github.com/jakubpawlowicz/clean-css/blob/master/bin/cleancss each time a new option is in. Should not we switch just to passing JS-named-options through? (with corresponding exceptions of course)).