Guest author links should be filterable

With two small, but significant, exceptions, the code in `CoAuthors_Guest_Authors:filter_author_link()` is the same as the function it's trying to filter, `get_author_posts_url()`.

The first difference, is where the `user_nicename` is set for guest authors. Without it, guest author links aren't appropriately created.

The other difference, is the lack of the `author_link` filter. Anyone trying to manipulate links returned by `get_author_posts_url()` or use a non-standard rewrite tag in WP_Rewrite::author_base, will find that they're filters fail. Found this while try to track down an issue a user was having with my [Edit Author Slug](https://github.com/thebrandonallen/edit-author-slug/commits/peter-debug) plugin.

This pull request keeps us DRY, and maintains the `author_link` filter on guest authors.