Proper hierarchy for the footer option

It's a trivial thing, but I've just had to look at the source of `concat.js` to find the footer option. This was _after_ reading the readme, and concluding this option didn't exist!

Why did I miss it? The hierarchy previously given to this option made it look as if footer was somehow a property of banner, when they really are **sibling options** from the end user's POV.