Option to not hijack core book links

This would help when other modules try to adjust the core book links.

For example BookMadeSimple adds a permission to 'show core Outline links', but it doesn't work when Outline module is enabled because the core $links['book_add_child'] are replaced by $links['outline_add_child'].

With a simple configuration this feature could be optionally disabled, while still keeping the rest of Outline's functionality.