New Filters: previous_posts_link_attributes and next_posts_link_attributes

[JohnLamansky]

I'm attaching a patch containing some simple enhancements to the previous/next posts link functions.

1. New "get_next_posts" and "get_previous_posts" functions (which are non-echo versions of "next_posts" and "previous_posts"). The rationale behind this is that a plain URL would probably need to be returned for PHP use more often than echo'd.

2. New "next_posts_link" and "previous_posts_link" filters -- useful for adding class, rel, or title attributes to the previous/next post links.

A less-convoluted way of allowing attributes; use this instead.