Allow modify ExtraWidth value for Supersubs

I have problem using superfish and IconApi module. The Supersubs don't determine the real width of li element and ::before element (font icon).

The situation can be mitigated set ExtraWidth value to more than 1.

Now the simplest solution is to modify it in superseubs.js default. A better solution is to modify it in the same way that min and max width for supersubs.

The better solution is to determine correctly element width with ::before and ::after elements, if any.