adds default flag, so users can set it from outside the file

Hi @appleboy!

If you add the `!default` flag to your variables, developers are able to set this variable from outside the file, so they don't have to meddle with the source of the component [this article on Sass components for details](see (http://fettblog.eu/blog/2014/01/13/manageable-sass-components/). 

Would be great if you could add it :-)