Added optional subfield argument to reference search

lib/podio/models/reference.rb

@@ -21,7 +21,7 @@ def find(ref_type, ref_id, options = {})
     end
 
     # @see https://developers.podio.com/doc/reference/search-references-13312595
-    def search(target, query, limit, target_params = nil)
+    def search(target, query, limit, target_params = nil, subfield = nil)
       response = Podio.connection.post do |req|
         req.url "/reference/search"
         req.body = {
@@ -30,6 +30,7 @@ def search(target, query, limit, target_params = nil)
           :limit => limit
         }
         req.body[:target_params] = target_params if target_params.present?
+        req.body[:subfield] = subfield if subfield.present?
       end
       response.body
     end