Latest News: allow more customized content

[pkevan]

The lastest news block currently only allows blog_id and items to show as part of it - we should allow further element to be added either within code, or within the admin.

In it's current use on the wp.org homepage, we grab the latest posts from /news, but due to the current limitations it only grabs from post post type, which doesn't accurately reflect the content on /news when you visit (/news adds in the podcast post type), so you end up with a jarring experience between the two.

[adamwoodnz]

Note that this block is also used on https://developer.wordpress.org/ for the Developer Blog section.