Allow for sorting by post field or meta value

[mogmarsh]

Description

It could be useful to sort posts in a query block by a post field meta value. The meta field options should be filterable by php - with a meta_key and a display name.
We should be able to support ascending or descending.

Use Case

In the case of content that is not "posts", we may want to order by the title or by a meta field with a separate date from the publish date.