interp: add support for extglob

[andreynering]

I know that it was said that this is not a priority on this comment:

The interpreter doesn't support expanding that syntax, simply because this feature is very rarely used, so it's not been a priority.

But have scripts that already uses extglob that we wanted to run with the interpreter, and since I didn't see any other opened issue about it, I decided to open this one to at least track this as a feature request.