added an option to discard importing stss rules from imports

[xavierlacot]

Basically, this option allows to only import variables defined in imported stss files, I have a case where this is useful.

[RonaldTreur]

Hi @xavierlacot. What is stopping you from simply extracting the variables into a separate file that you then include in the old file?

I'm not really sure about this. The way it currently works is similar to how SCSS is parsed. That is by design, since I'm assuming users will expect a SCSS's behavior.

However, if others find this useful as well I'll be sure to add it.