Create has_author conditional

[jonathanstegall]

It doesn't have to happen for this initial new version, but I'd like to support a has_author conditional. With it, we would supply a parameter of one or more authors that exist via the Co-Authors plugin, and the conditional would check against the authors of the current post to see if there was a match. Basically the same as the already existing has_category conditional.

This only needs to run on single posts, or other content types that use Co-Authors. We already have the ability to check is_author for author archives.