Handle printing press signature options intentionally

[alerque]

Add explicit signature (printing press layout / sheet folding) handling for books on a per-format basis. 4/8/16/32 sheet, blank page content, warnings about optimizing for a specific signature, etc.

One thing I keep running into when doing booklets is that using a 32 page signature works out great for booklets that are about 30 pages anyway, but running over to 33 pages is just not an option. Either optional content to drop if an overrun is detected, a build time warning or error, or a formatting value to tweak and try again would be nice.