Fixed "extract" in order to account for other numbers in the slug

My site had an issue for slugs that already had a number in them. Say for example we had something called "24 Hour Fitness" which gave a slug of "24-hour-fitness". When we created a second item with the same name, we would get a slug of "24-hour-fitness--25". Our separator was "--", but the .split would return the entire string if it didn't find that separator (and it wouldn't if we only had one of these items). The conversion to integer would then find the 24 and think that was the last of the "sequence".

With this code, it will not convert unless it finds the separator. If it doesn't find the separator, there must just be a single one that already exists.

I know this isn't version 5, but it would be nice to have this for those of us who need to use the older stable version.