Extend submenutree with support for book.module

Thanks so much for that extra hook_submenutree_mlid()!
It's just right to add book support :-)

So, here's a mini-module that extends this module to do the submenu_tree magic on books.

Got a site structure that contains a load of books - for management reasons. And showing teasers of child pages is a nice thing - so nice that we are happy to ditch the normal book nav.

Getting the submenutree to use the book menu not the default menu was pretty simple, thanks to that hook. Slightly messier was getting book.module not to show its normal links, but it's still better doing it here in a module rather than putting logic in the theme.

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