Disable/hide email button for authors 

who haven't published in the previous six months.

Applies to https://github.com/GigaOM/gigaom/issues/4939
I have mixed feelings about moving this into a separate method. I'm not sure what it adds that WP_Query doesn't offer. 

We definitely _don't_ want to use this method instead of WP_Query to get posts.

We definitely need to do sanity checking that isn't happening either here or in the calling method, and in doing that we'll have to decide if we want to sanity check that we have a generic post, or narrowly sanity check that we have just the date on a post.

Since we're not trying to build a generic post getter (WP already provides one with WP_Query), we might be better off simply calling WP_Query above and _not_ creating a new method.