Allow filtering post-type archive rewrites

[ethitter]

If one wishes to introduce additional query arguments into a post-type archive's rewrite structure, the current options are rather cumbersome: filter the rewrites as they're generated, filter the whole rewrites array, or reproduce a considerable amount of logic from WP_Post_Type::add_rewrite_rules(). By introducing a few filters on the archive-specific rewrites, it becomes far easier to customize a post type's archive rewrites.