New function for mutate filter


it merges two fields or arrays or hashes in the first one
Strings fields will be converted in array, so 
- array + string will work
- string + string will result in an 2 entry array in dest_field
- array and hash will not work

examples
mutate { 
               # tags_field = "tag1,tag2,tag3"

+  #          merge => ["@tags", "tags_fields"]
+  #       }
+  #     }