Support flexible pagination configuration

This review builds on review #654, and includes the changes from it, rebased to master. Instead of a single pagination URL/SAVE_AS setting, however, it allows authors to define a set of rules to be used based on the page index. This allows to create URLs that look like:

/blog/    # page 1
/blog/page/2/   # page 2

The settings I use to generate those URLs are

    PAGINATION_PATTERNS = (
        (1, '{base_name}/', '{base_name}/index.html'),
        (2, '{base_name}/page/{number}/', '{base_name}/page/{number}/index.html'),
    )