Adds date range support to related posts query

Adds a filterable date range to Related Posts, to prevent overly stale
content from being returned.
Prepare database filters for extra security

Just to be safe, make sure the filtered $where clause is run through
$wpdb->prepare()
Merge pull request #13 from nickdaugherty/master

Adds date range to ES and DB queries, with a -1 year default, to prevent overly stale data from being presented as related